PHP: Lo básico

Lo básico

class

La definición básica de una clase comienza con la palabra reservada class, seguida de un nombre de clase, y continuando con un par de llaves que encierran las definiciones de las propiedades y métodos pertenecientes a dicha clase.

El nombre de clase puede ser cualquier etiqueta válida, siempre que no sea una palabra reservada de PHP. Un nombre válido de clase comienza con una letra o un guión bajo, seguido de una cantidad arbitraria de letras, números o guiones bajos. Como expresión regular, se expresaría de la siguiente forma: ^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$.

Una clase puede tener sus propias constantes, variables (llamadas "propiedades"), y funciones (llamados "métodos").

Ejemplo #1 Definición de una clase sencilla

<?php
class ClaseSencilla
{
    // Declaración de una propiedad
    public $var = 'un valor predeterminado';

    // Declaración de un método
    public function mostrarVar() {
        echo $this->var;
    }
}
?>

La pseudovariable $this está disponible cuando un método es invocado dentro del contexto de un objeto. $this es una referencia al objeto invocador.

Advertencia

Llamar a un método no estático arroja estáticamente un Error. Antes de PHP 8.0.0, esto generaría un aviso de desaprobación obsoleta, y $this esto no estaría definido.

Ejemplo #2 Algunos ejemplos de la pseudovariable $this

<?php
class A
{
    function foo()
    {
        if (isset($this)) {
            echo '$this está definida (';
            echo get_class($this);
            echo ")\n";
        } else {
            echo "\$this no está definida.\n";
        }
    }
}

class B
{
    function bar()
    {
        A::foo();
    }
}

$a = new A();
$a->foo();

A::foo();

$b = new B();
$b->bar();

B::bar();
?>

Salida del ejemplo anterior en PHP 7:

$this está definida (A)

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 27
$this is not defined.

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 20
$this is not defined.

Deprecated: Non-static method B::bar() should not be called statically in %s  on line 32

Deprecated: Non-static method A::foo() should not be called statically in %s  on line 20
$this is not defined.

Output of the above example in PHP 8:

$this está definida (A)

Fatal error: Uncaught Error: Non-static method A::foo() cannot be called statically in %s :27
Stack trace:
#0 {main}
  thrown in %s  on line 27

Readonly classes

A partir de PHP 8.2.0, una clase puede ser marcada con el modificador readonly. Marcar una clase como readonly añadirá el readonly modificador a cada propiedad declarada, y evitará la creación de propiedades dinámicas. Además, es imposible añadir soporte para ellas utilizando el atributo AllowDynamicProperties. Intentar hacerlo provocará un error en tiempo de compilación.

<?php
#[\AllowDynamicProperties]
readonly class Foo {
}

// Fatal error: Cannot apply #[AllowDynamicProperties] to readonly class Foo
?>

Daado que ni las propiedades no tipadas ni las estáticas pueden ser marcadas con el modificador readonly, las clases readonly tampoco pueden ser declararlas:

<?php
readonly class Foo
{
    public $bar;
}

// Fatal error: Readonly property Foo::$bar must have type
?>

<?php
readonly class Foo
{
    public static int $bar;
}

// Fatal error: Readonly class Foo cannot declare static properties
?>

Una clase readonly puede ser extendida si, y solo si, la clase hija también es una clase readonly.

new

Para crear una instancia de una clase, se debe emplear la palabra reservada new. Un objeto se creará siempre a menos que el objeto tenga un constructor que arroje una excepción en caso de error. Las clases deberían ser definidas antes de la instanciación (y en algunos casos esto es un requerimiento).

Si se emplea un string que contenga el nombre de una clase con new, se creará una nueva instancia de esa clase. Si la clase estuviera en un espacio de nombres, se debe utilizar su nombre completo al realizar esto.

Nota:
Si no hay argumentos para pasar al constructor de la clase, se pueden omitir los paréntesis después del nombre de la clase.

Ejemplo #3 Creación de una instancia

<?php
$instancia = new ClaseSencilla();

// Esto también se puede hacer con una variable:
$nombreClase = 'ClaseSencilla';
$instancia = new $nombreClase(); // new ClaseSencilla()
?>

En el contexto de una clase, es posible crear un nuevo objeto con new self y new parent.

Cuando se asigna una instancia ya creada de una clase a una nueva variable, ésta última accederá a la misma instancia que el objeto que le fue asignado. Esta conducta es la misma que cuando se pasan instancias a una función. Se puede realizar una copia de un objeto ya creado a través de la clonación del mismo.

Ejemplo #4 Asignación de objetos

<?php

$instancia = new ClaseSencilla();

$asignada   =  $instancia;
$referencia =& $instancia;

$instancia->var = '$asignada tendrá este valor';

$instancia = null; // $instancia y $referencia son null

var_dump($instancia);
var_dump($referencia);
var_dump($asignada);
?>

El resultado del ejemplo sería:

NULL
NULL
object(ClaseSencilla)#1 (1) {
   ["var"]=>
     string(27) "$asignada tendrá este valor"
}

Existe un par de formas de crear instancias de un objecto:

Ejemplo #5 Creación de nuevos objetos

<?php
class Prueba
{
    static public function getNew()
    {
        return new static;
    }
}

class Hija extends Prueba
{}

$obj1 = new Prueba();
$obj2 = new $obj1;
var_dump($obj1 !== $obj2);

$obj3 = Prueba::getNew();
var_dump($obj3 instanceof Prueba);

$obj4 = Hija::getNew();
var_dump($obj4 instanceof Hija);
?>

El resultado del ejemplo sería:

bool(true)
bool(true)
bool(true)

Es posible acceder a un miembro de un objeto recién creado en una única expresión:

Ejemplo #6 Acceder a un miembro de un objeto recién creado

<?php
echo (new DateTime())->format('Y');
?>

El resultado del ejemplo sería algo similar a:

Nota: Antes de PHP 7.1, los argumentos no se evalúan si no hay una función constructora definida.

Propiedades y métodos

Las propiedades y métodos de una clase viven en «espacios de nombres» diferentes, por tanto, es posible tener una propiedad y un método con el mismo nombre. Al hacer referencia tanto a una propiedad como a un método se utiliza la misma notación, y si se accederá a la propiedad o se llamará al método, solamente depende del contexto, es decir, si el empleo es el acceso a una variable o la llamada a una función.

Ejemplo #7 Acceso a propiedad contra llamada a método

<?php
class Foo
{
    public $bar = 'property';

    public function bar() {
        return 'method';
    }
}

$obj = new Foo();
echo $obj->bar, PHP_EOL, $obj->bar(), PHP_EOL;

El resultado del ejemplo sería:

propiedad
método

Esto significa que llamar a una función anónima que ha sido asignada a una propiedad no es posible directamente. En su lugar, la propiedad ha de ser asignada primero a una variable, por ejemplo. A partir de PHP 7.0.0, es posible llamar a dicha propiedad directamente encerrándola entre paréntesis.

Ejemplo #8 Llamar a una función anónima almacenada en una propiedad

<?php
class Foo
{
    public $bar;

    public function __construct() {
        $this->bar = function() {
            return 42;
        };
    }
}

$obj = new Foo();

echo ($obj->bar)(), PHP_EOL;

El resultado del ejemplo sería:

extends

Una clase puede heredar los métodos y propiedades de otra clase empleando la palabra reservada extends en la declaración de la clase. No es posible la extensión de múltiples clases; una clase sólo puede heredar de una clase base.

Los métodos y propiedades heredados pueden ser sobrescritos con la redeclaración de éstos utilizando el mismo nombre que en la clase madre. Sin embargo, si la clase madre definió un método como final, éste no podrá ser sobrescrito. Es posible acceder a los métodos sobrescritos o a las propiedades estáticas haciendo referencia a ellos con parent::.

Ejemplo #9 Herencia de clases sencilla

<?php
class ClaseExtendida extends ClaseSencilla
{
    // Redefinición del método padre
    function mostrarVar()
    {
        echo "Clase extendida\n";
        parent::mostrarVar();
    }
}

$extendida = new ClaseExtendida();
$extendida->mostrarVar();
?>

El resultado del ejemplo sería:

Clase extendida
un valor predeterminado

Signature compatibility rules

When overriding a method, its signature must be compatible with the parent method. Otherwise, a fatal error is emitted, or, prior to PHP 8.0.0, an E_WARNING level error is generated. A signature is compatible if it respects the variance rules, makes a mandatory parameter optional, and if any new parameters are optional. This is known as the Liskov Substitution Principle, or LSP for short. The constructor, and private methods are exempt from these signature compatibility rules, and thus won't emit a fatal error in case of a signature mismatch.

Ejemplo #10 Compatible child methods

<?php

class Base
{
    public function foo(int $a) {
        echo "Valid\n";
    }
}

class Extend1 extends Base
{
    function foo(int $a = 5)
    {
        parent::foo($a);
    }
}

class Extend2 extends Base
{
    function foo(int $a, $b = 5)
    {
        parent::foo($a);
    }
}

$extended1 = new Extend1();
$extended1->foo();
$extended2 = new Extend2();
$extended2->foo(1);

El resultado del ejemplo sería:

Valid
Valid

The following examples demonstrate that a child method which removes a parameter, or makes an optional parameter mandatory, is not compatible with the parent method.

Ejemplo #11 Fatal error when a child method removes a parameter

<?php

class Base
{
    public function foo(int $a = 5) {
        echo "Valid\n";
    }
}

class Extend extends Base
{
    function foo()
    {
        parent::foo(1);
    }
}

Output of the above example in PHP 8 is similar to:

Fatal error: Declaration of Extend::foo() must be compatible with Base::foo(int $a = 5) in /in/evtlq on line 13

Ejemplo #12 Fatal error when a child method makes an optional parameter mandatory

<?php

class Base
{
    public function foo(int $a = 5) {
        echo "Valid\n";
    }
}

class Extend extends Base
{
    function foo(int $a)
    {
        parent::foo($a);
    }
}

Output of the above example in PHP 8 is similar to:

Fatal error: Declaration of Extend::foo(int $a) must be compatible with Base::foo(int $a = 5) in /in/qJXVC on line 13

Advertencia

Renaming a method's parameter in a child class is not a signature incompatibility. However, this is discouraged as it will result in a runtime Error if named arguments are used.

Ejemplo #13 Error when using named arguments and parameters were renamed in a child class

<?php

class A {
    public function test($foo, $bar) {}
}

class B extends A {
    public function test($a, $b) {}
}

$obj = new B;

// Pass parameters according to A::test() contract
$obj->test(foo: "foo", bar: "bar"); // ERROR!

El resultado del ejemplo sería algo similar a:

Fatal error: Uncaught Error: Unknown named parameter $foo in /in/XaaeN:14
Stack trace:
#0 {main}
  thrown in /in/XaaeN on line 14

::class

La palabra reservada class es usada para la resolución de nombres de clases. Se puede obtener un string con el nombre completamente cualificado de la clase ClassName utilizando ClassName::class. Esto es particularmete útil con clases en espacios de nombres.

Ejemplo #14 Resolución de nombres de clases

<?php
namespace NS {
    class ClassName {
    }

    echo ClassName::class;
}
?>

El resultado del ejemplo sería:

NS\ClassName

Nota:
La resolución de nombres de clases con ::class es una transformación durante la compilación. Esto significa que, en el instante de crear el string del nombre de la clase, aún no se ha realizado ninguna autocarga. Como consecuencia, los nombres de clases se expanden incluso si la clase no existe. No se emite ningún error en tal caso.
Ejemplo #15 Missing class name resolution

<?php print Does\Not\Exist::class; ?>

El resultado del ejemplo sería:
Does\Not\Exist

As of PHP 8.0.0, the ::class constant may also be used on objects. This resolution happens at runtime, not compile time. Its effect is the same as calling get_class() on the object.

Ejemplo #16 Object name resolution

<?php
namespace NS {
    class ClassName {
    }
}
$c = new ClassName();
print $c::class;
?>

El resultado del ejemplo sería:

NS\ClassName

Nullsafe methods and properties

As of PHP 8.0.0, properties and methods may also be accessed with the "nullsafe" operator instead: ?->. The nullsafe operator works the same as property or method access as above, except that if the object being dereferenced is null then null will be returned rather than an exception thrown. If the dereference is part of a chain, the rest of the chain is skipped.

The effect is similar to wrapping each access in an is_null() check first, but more compact.

Ejemplo #17 Nullsafe Operator

<?php

// As of PHP 8.0.0, this line:
$result = $repository?->getUser(5)?->name;

// Is equivalent to the following code block:
if (is_null($repository)) {
    $result = null;
} else {
    $user = $repository->getUser(5);
    if (is_null($user)) {
        $result = null;
    } else {
        $result = $user->name;
    }
}
?>

Nota:
The nullsafe operator is best used when null is considered a valid and expected possible value for a property or method return. For indicating an error, a thrown exception is preferable.

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User Contributed Notes 11 notes

down

641

aaron at thatone dot com ¶

17 years ago

I was confused at first about object assignment, because it's not quite the same as normal assignment or assignment by reference. But I think I've figured out what's going on.

First, think of variables in PHP as data slots. Each one is a name that points to a data slot that can hold a value that is one of the basic data types: a number, a string, a boolean, etc. When you create a reference, you are making a second name that points at the same data slot. When you assign one variable to another, you are copying the contents of one data slot to another data slot.

Now, the trick is that object instances are not like the basic data types. They cannot be held in the data slots directly. Instead, an object's "handle" goes in the data slot. This is an identifier that points at one particular instance of an obect. So, the object handle, although not directly visible to the programmer, is one of the basic datatypes. 

What makes this tricky is that when you take a variable which holds an object handle, and you assign it to another variable, that other variable gets a copy of the same object handle. This means that both variables can change the state of the same object instance. But they are not references, so if one of the variables is assigned a new value, it does not affect the other variable.

<?php
// Assignment of an object
Class Object{
   public $foo="bar";
};

$objectVar = new Object();
$reference =& $objectVar;
$assignment = $objectVar

//
// $objectVar --->+---------+
//                |(handle1)----+
// $reference --->+---------+   |
//                              |
//                +---------+   |
// $assignment -->|(handle1)----+
//                +---------+   |
//                              |
//                              v
//                  Object(1):foo="bar"
//
?>

$assignment has a different data slot from $objectVar, but its data slot holds a handle to the same object. This makes it behave in some ways like a reference. If you use the variable $objectVar to change the state of the Object instance, those changes also show up under $assignment, because it is pointing at that same Object instance.

<?php
$objectVar->foo = "qux";
print_r( $objectVar );
print_r( $reference );
print_r( $assignment );

//
// $objectVar --->+---------+
//                |(handle1)----+
// $reference --->+---------+   |
//                              |
//                +---------+   |
// $assignment -->|(handle1)----+
//                +---------+   |
//                              |
//                              v
//                  Object(1):foo="qux"
//
?>

But it is not exactly the same as a reference. If you null out $objectVar, you replace the handle in its data slot with NULL. This means that $reference, which points at the same data slot, will also be NULL. But $assignment, which is a different data slot, will still hold its copy of the handle to the Object instance, so it will not be NULL.

<?php
$objectVar = null;
print_r($objectVar);
print_r($reference);
print_r($assignment);

//
// $objectVar --->+---------+
//                |  NULL   | 
// $reference --->+---------+
//                           
//                +---------+
// $assignment -->|(handle1)----+
//                +---------+   |
//                              |
//                              v
//                  Object(1):foo="qux"
?>

down

kStarbe at gmail point com ¶

7 years ago

You start using :: in second example although the static concept has not been explained. This is not easy to discover when you are starting from the basics.

down

129

Doug ¶

14 years ago

What is the difference between  $this  and  self ?

Inside a class definition, $this refers to the current object, while  self  refers to the current class.

It is necessary to refer to a class element using  self ,
and refer to an object element using  $this .
Note also how an object variable must be preceded by a keyword in its definition.

The following example illustrates a few cases:

<?php
class Classy {

const       STAT = 'S' ; // no dollar sign for constants (they are always static)
static     $stat = 'Static' ;
public     $publ = 'Public' ;
private    $priv = 'Private' ;
protected  $prot = 'Protected' ;

function __construct( ){  }

public function showMe( ){
    print '<br> self::STAT: '  .  self::STAT ; // refer to a (static) constant like this
    print '<br> self::$stat: ' . self::$stat ; // static variable
    print '<br>$this->stat: '  . $this->stat ; // legal, but not what you might think: empty result
    print '<br>$this->publ: '  . $this->publ ; // refer to an object variable like this
    print '<br>' ;
}
}
$me = new Classy( ) ;
$me->showMe( ) ;

/* Produces this output:
self::STAT: S
self::$stat: Static
$this->stat:
$this->publ: Public
*/
?>

down

Hayley Watson ¶

6 years ago

Class names are case-insensitive:
<?php
class Foo{}
class foo{} //Fatal error.
?>

Any casing can be used to refer to the class
<?php
class bAr{}
$t = new Bar();
$u = new bar();
echo ($t instanceof $u) ? "true" : "false"; // "true"
echo ($t instanceof BAR) ? "true" : "false"; // "true"
echo is_a($u, 'baR') ? "true" : "false"; // "true"
?>

But the case used when the class was defined is preserved as "canonical":
<?php
echo get_class($t); // "bAr"
?>

And, as always, "case-insensitivity" only applies to ASCII.
<?php
class пасха{}
class Пасха{} // valid
$p = new ПАСХА(); // Uncaught warning.
?>

down

wbcarts at juno dot com ¶

16 years ago

CLASSES and OBJECTS that represent the "Ideal World"

Wouldn't it be great to get the lawn mowed by saying $son->mowLawn()? Assuming the function mowLawn() is defined, and you have a son that doesn't throw errors, the lawn will be mowed. 

In the following example; let objects of type Line3D measure their own length in 3-dimensional space. Why should I or PHP have to provide another method from outside this class to calculate length, when the class itself holds all the neccessary data and has the education to make the calculation for itself?

<?php

/*
 * Point3D.php
 *
 * Represents one locaton or position in 3-dimensional space
 * using an (x, y, z) coordinate system.
 */
class Point3D
{
    public $x;
    public $y;
    public $z;                  // the x coordinate of this Point.

    /*
     * use the x and y variables inherited from Point.php.
     */
    public function __construct($xCoord=0, $yCoord=0, $zCoord=0)
    {
        $this->x = $xCoord;
    $this->y = $yCoord;
        $this->z = $zCoord;
    }

    /*
     * the (String) representation of this Point as "Point3D(x, y, z)".
     */
    public function __toString()
    {
        return 'Point3D(x=' . $this->x . ', y=' . $this->y . ', z=' . $this->z . ')';
    }
}

/*
 * Line3D.php
 *
 * Represents one Line in 3-dimensional space using two Point3D objects.
 */
class Line3D
{
    $start;
    $end;

    public function __construct($xCoord1=0, $yCoord1=0, $zCoord1=0, $xCoord2=1, $yCoord2=1, $zCoord2=1)
    {
        $this->start = new Point3D($xCoord1, $yCoord1, $zCoord1);
        $this->end = new Point3D($xCoord2, $yCoord2, $zCoord2);
    }

    /*
     * calculate the length of this Line in 3-dimensional space.
     */ 
    public function getLength()
    {
        return sqrt(
            pow($this->start->x - $this->end->x, 2) +
            pow($this->start->y - $this->end->y, 2) +
            pow($this->start->z - $this->end->z, 2)
        );
    }

    /*
     * The (String) representation of this Line as "Line3D[start, end, length]".
     */
    public function __toString()
    {
        return 'Line3D[start=' . $this->start .
            ', end=' . $this->end .
            ', length=' . $this->getLength() . ']';
    }
}

/*
 * create and display objects of type Line3D.
 */
echo '<p>' . (new Line3D()) . "</p>\n";
echo '<p>' . (new Line3D(0, 0, 0, 100, 100, 0)) . "</p>\n";
echo '<p>' . (new Line3D(0, 0, 0, 100, 100, 100)) . "</p>\n";

?>

  <--  The results look like this  -->

Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=1, y=1, z=1), length=1.73205080757]

Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=100, y=100, z=0), length=141.421356237]

Line3D[start=Point3D(x=0, y=0, z=0), end=Point3D(x=100, y=100, z=100), length=173.205080757]

My absolute favorite thing about OOP is that "good" objects keep themselves in check. I mean really, it's the exact same thing in reality... like, if you hire a plumber to fix your kitchen sink, wouldn't you expect him to figure out the best plan of attack? Wouldn't he dislike the fact that you want to control the whole job? Wouldn't you expect him to not give you additional problems? And for god's sake, it is too much to ask that he cleans up before he leaves?

I say, design your classes well, so they can do their jobs uninterrupted... who like bad news? And, if your classes and objects are well defined, educated, and have all the necessary data to work on (like the examples above do), you won't have to micro-manage the whole program from outside of the class. In other words... create an object, and LET IT RIP!

down

moty66 at gmail dot com ¶

15 years ago

I hope that this will help to understand how to work with static variables inside a class

<?php

class a {

    public static $foo = 'I am foo';
    public $bar = 'I am bar';
    
    public static function getFoo() { echo self::$foo;    }
    public static function setFoo() { self::$foo = 'I am a new foo'; }
    public function getBar() { echo $this->bar;    }            
}

$ob = new a();
a::getFoo();     // output: I am foo    
$ob->getFoo();    // output: I am foo
//a::getBar();     // fatal error: using $this not in object context
$ob->getBar();    // output: I am bar
                // If you keep $bar non static this will work
                // but if bar was static, then var_dump($this->bar) will output null 

// unset($ob);
a::setFoo();    // The same effect as if you called $ob->setFoo(); because $foo is static
$ob = new a();     // This will have no effects on $foo
$ob->getFoo();    // output: I am a new foo 

?>

Regards
Motaz Abuthiab

down

pawel dot zimnowodzki at gmail dot com ¶

2 years ago

Although there is no null-safe operator for not existed array keys I found workaround for it: ($array['not_existed_key'] ?? null)?->methodName()

down

Notes on stdClass ¶

15 years ago

stdClass is the default PHP object. stdClass has no properties, methods or parent. It does not support magic methods, and implements no interfaces.

When you cast a scalar or array as Object, you get an instance of stdClass. You can use stdClass whenever you need a generic object instance.
<?php
// ways of creating stdClass instances
$x = new stdClass;
$y = (object) null;        // same as above
$z = (object) 'a';         // creates property 'scalar' = 'a'
$a = (object) array('property1' => 1, 'property2' => 'b');
?>

stdClass is NOT a base class! PHP classes do not automatically inherit from any class. All classes are standalone, unless they explicitly extend another class. PHP differs from many object-oriented languages in this respect.
<?php
// CTest does not derive from stdClass
class CTest {
    public $property1;
}
$t = new CTest;
var_dump($t instanceof stdClass);            // false
var_dump(is_subclass_of($t, 'stdClass'));    // false
echo get_class($t) . "\n";                   // 'CTest'
echo get_parent_class($t) . "\n";            // false (no parent)
?>

You cannot define a class named 'stdClass' in your code. That name is already used by the system. You can define a class named 'Object'.

You could define a class that extends stdClass, but you would get no benefit, as stdClass does nothing.

(tested on PHP 5.2.8)

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johannes dot kingma at gmail dot com ¶

3 years ago

BEWARE! 

Like Hayley Watson pointed out class names are not case sensitive. 

<?php
class Foo{}
class foo{} // Fatal error: Cannot declare class foo, because the name is already in use
?>
As well as
<?php
class BAR{}
$bar = new Bar();
echo get_class($bar);
?> 

Is perfectly fine and will return 'BAR'.

This has implications on autoloading classes though. The standard spl_autoload function will strtolower the class name to cope with case in-sensitiveness and thus the class BAR can only be found if the file name is bar.php (or another variety if an extension was registered with spl_autoload_extensions(); ) not BAR.php for a case sensitive file and operating system like linux. Windows file system is case sensitive but the OS is not  and there for autoloading BAR.php will work.

down

Jeffrey ¶

16 years ago

A PHP Class can be used for several things, but at the most basic level, you'll use classes to "organize and deal with like-minded data". Here's what I mean by "organizing like-minded data". First, start with unorganized data.

<?php
$customer_name;
$item_name;
$item_price;
$customer_address;
$item_qty;
$item_total;
?>

Now to organize the data into PHP classes:

<?php
class Customer {
  $name;          // same as $customer_name
  $address;       // same as $customer_address
}

class Item {
  $name;          // same as $item_name
  $price;         // same as $item_price
  $qty;           // same as $item_qty
  $total;         // same as $item_total
}
?>

Now here's what I mean by "dealing" with the data. Note: The data is already organized, so that in itself makes writing new functions extremely easy.

<?php
class Customer {
  public $name, $address;                   // the data for this class...

  // function to deal with user-input / validation
  // function to build string for output
  // function to write -> database
  // function to  read <- database
  // etc, etc
}

class Item {
  public $name, $price, $qty, $total;        // the data for this class...

  // function to calculate total
  // function to format numbers
  // function to deal with user-input / validation
  // function to build string for output
  // function to write -> database
  // function to  read <- database
  // etc, etc
}
?>

Imagination that each function you write only calls the bits of data in that class. Some functions may access all the data, while other functions may only access one piece of data. If each function revolves around the data inside, then you have created a good class.

down

Anonymous ¶

6 years ago

At first I was also confused by the assignment vs referencing but here's how I was finally able to get my head around it. This is another example which is somewhat similar to one of the comments but can be helpful to those who did not understand the first example. Imagine object instances as rooms where you can store and manipulate your properties and functions.  The variable that contains the object simply holds 'a key' to this room and thus access to the object. When you assign this variable to another new variable, what you are doing is you're making a copy of the key and giving it to this new variable. That means these two variable now have access to the same 'room' (object) and can thus get in and manipulate the values. However, when you create a reference, what you doing is you're making the variables SHARE the same key. They both have access to the room. If one of the variable is given a new key, then the key that they are sharing is replaced and they now share a new different key. This does not affect the other variable with a copy of the old key...that variable still has access to the first room

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