If your code looks like this:<?php    namespace NS;?>...and you still get "Namespace declaration statement has to be the very first statement in the script" Fatal error, then you probably use UTF-8 encoding (which is good) with Byte Order Mark, aka BOM (which is bad). Try to convert your files to "UTF-8 without BOM", and it should be ok.

Regarding constants defined with define() inside namespaces...define() will define constants exactly as specified.  So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace.  The following examples will make it clear.The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").<?phpnamespace test;define('MESSAGE', 'Hello world!');?>The following code will define two constants in the "test" namespace.<?phpnamespace test;define('test\HELLO', 'Hello world!');define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!');?>

Expanding on @danbettles note, it is better to always be explicit about which constant to use.<?php    namespace NS;    define(__NAMESPACE__ .'\foo','111');    define('foo','222');    echo foo;  // 111.    echo \foo;  // 222.    echo \NS\foo  // 111.    echo NS\foo  // fatal error. assumes \NS\NS\foo.?>

namespace statement  is defined at first of the php files. But     before namespace declaration only three elements allowed.      1.declare statement      2.spaces      3.comments

"A file containing a namespace must declare the namespace at the top of the file before any other code"

It might be obvious, but this means that you *can* include comments and white spaces before the namespace keyword.

<?php
// Lots 
// of
// interesting
// comments and white space

namespace Foo;
class Bar {
}
?>

You should not try to create namespaces that use PHP keywords. These will cause parse errors. 

Examples:

<?php
namespace Project/Classes/Function; // Causes parse errors
namespace Project/Abstract/Factory; // Causes parse errors
?>

@ RS: Also, you can specify how your __autoload() function looks for the files. That way another users namespace classes cannot overwrite yours unless they replace your file specifically.

As of PHP 8.1, not only classes (including abstracts and traits), interfaces, functions, and constants are affected by namespaces, but also enums.<?phpnamespace App\Types;enum Colors {    case Red;    case Green;}$reflection = new \ReflectionEnum('App\Types\Colors');var_dump($reflection->getName()); // string(16) "App\Types\Colors"var_dump($reflection->getNamespaceName()); // string(9) "App\Types"?>

There is nothing wrong with PHP namespaces, except that those 2 instructions give a false impression of package management.... while they just correspond to the "with()" instruction of Javascript.By contrast, a package is a namespace for its members, but it offers more (like deployment facilities), and a compiler knows exactly what classes are in a package, and where to find them.

Namespace name are case-insensitive.namespace Appandnamespace appare same meaning.Besides, Namespace keword are case-insensitive.Namespace Appnamespace AppandNAMESPACE Appare same meaning.

Please note that a PHP Namespace declaration cannot start with a number.It took some time for me to debug...