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Note that even though PHP borrows large portions of its syntax from C, the ',' is treated quite differently. It's not possible to create combined expressions in PHP using the comma-operator that C has, except in for() loops.Example (parse error):<?php$a = 2, $b = 4;echo $a."\n";echo $b."\n";?>Example (works):<?phpfor ($a = 2, $b = 4; $a < 3; $a++){  echo $a."\n";  echo $b."\n";}?>This is because PHP doesn't actually have a proper comma-operator, it's only supported as syntactic sugar in for() loop headers. In C, it would have been perfectly legitimate to have this:int f(){  int a, b;  a = 2, b = 4;  return a;}or even this:int g(){  int a, b;  a = (2, b = 4);  return a;}In f(), a would have been set to 2, and b would have been set to 4.In g(), (2, b = 4) would be a single expression which evaluates to 4, so both a and b would have been set to 4.

Manual defines "expression is anything that has value", Therefore, parser will give error for following code.

<?php
($val) ? echo('true') : echo('false');
Note: "? : " operator has this syntax  "expr ? expr : expr;"
?>

since echo does not have(return) value and ?: expects expression(value).

However, if function/language constructs that have/return value, such as include(), parser compiles code.

Note: User defined functions always have/return value without explicit return statement (returns NULL if there is no return statement). Therefore, user defined functions are always valid expressions. 
[It may be useful to have VOID as new type to prevent programmer to use function as RVALUE by mistake]

For example,

<?php
($val) ? include('true.inc') : include('false.inc');
?>

is valid, since "include" returns value.

The fact "echo" does not return value(="echo" is not a expression), is less obvious to me. 

Print() and Echo() is NOT identical since print() has/returns value and can be a valid expression.

Note that there is a difference between a function and a function call, and bothare expressions. PHP has two kinds of function, "named functions" and "anonymousfunctions". Here's an example with both:<?php// A named function. Its name is "double".function double($x) {  return 2 * $x;}// An anonymous function. It has no name, in the same way that the string// "hello" has no name. Since it is an expression, we can give it a temporary// name by assigning it to the variable $triple.$triple = function($x) {  return 3 * $x;};?>We can "call" (or "run") both kinds of function. A "function call" is anexpression with the value of whatever the function returns. For example:<?php// The easiest way to run a function is to put () after its name, containing its// arguments (if any)$my_numbers = array(double(5), $triple(5));?>$my_numbers is now an array containing 10 and 15, which are the return values ofdouble and $triple when applied to the number 5.Importantly, if we *don't* call a function, ie. we don't put () after its name,then we still get expressions. For example:<?php$my_functions = array('double', $triple);?>$my_functions is now an array containing these two functions. Notice that namedfunctions are more awkward than anonymous functions. PHP treats them differentlybecause it didn't use to have anonymous functions, and the way named functionswere implemented didn't work for anonymous functions when they were eventuallyadded.This means that instead of using a named function literally, like we can withanonymous functions, we have to use a string containing its name instead. PHPmakes sure that these strings will be treated as functions when it'sappropriate. For example:<?php$temp      = 'double';$my_number = $temp(5);?>$my_number will be 10, since PHP has spotted that we're treating a string as ifit were a function, so it has looked up that named function for us.Unfortunately PHP's parser is very quirky; rather than looking for genericpatterns like "x(y)" and seeing if "x" is a function, it has lots ofspecial-cases like "$x(y)". This makes code like "'double'(5)" invalid, so wehave to do tricks like using temporary variables. There is another way aroundthis restriction though, and that is to pass our functions to the"call_user_func" or "call_user_func_array" functions when we want to call them.For example:<?php$my_numbers = array(call_user_func('double', 5), call_user_func($triple, 5));?>$my_numbers contains 10 and 15 because "call_user_func" called our functions forus. This is possible because the string 'double' and the anonymous function$triple are expressions. Note that we can even use this technique to call ananonymous function without ever giving it a name:<?php$my_number = call_user_func(function($x) { return 4 * $x; }, 5);?>$my_number is now 20, since "call_user_func" called the anonymous function,which quadruples its argument, with the value 5.Passing functions around as expressions like this is very useful whenever weneed to use a 'callback'. Great examples of this are array_map and array_reduce.

A note about the short-circuit behaviour of the boolean operators.1. if (func1() || func2())Now, if func1() returns true, func2() isn't run, since the expressionwill be true anyway.2. if (func1() && func2())Now, if func1() returns false, func2() isn't run, since the expressionwill be false anyway.The reason for this behaviour comes probably from the programminglanguage C, on which PHP seems to be based on. There theshort-circuiting can be a very useful tool. For example:int * myarray = a_func_to_set_myarray(); // init the arrayif (myarray != NULL && myarray[0] != 4321) // check    myarray[0] = 1234;Now, the pointer myarray is checked for being not null, then thecontents of the array is validated. This is important, because ifyou try to access an array whose address is invalid, the programwill crash and die a horrible death. But thanks to the shortcircuiting, if myarray == NULL then myarray[0] won't be accessed,and the program will work fine.

It is worthy to mention that:$n = 3;$n * --$nWILL RETURN 4 instead of 6.It can be a hard to spot "error", because in our human thought process this really isn't an error at all! But you have to remember that PHP (as it is with many other high-level languages) evaluates its statements RIGHT-TO-LEFT, and therefore "--$n" comes BEFORE multiplying, so - in the end - it's really "2 * 2", not "3 * 2".It is also worthy to mention that the same behavior will change:$n = 3;$n * $n++from 3 * 3 into 3 * 4. Post- operations operate on a variable after it has been 'checked', but it doesn't necessarily state that it should happen AFTER an evaluation is over (on the contrary, as a matter of fact).So, if you ever find yourself on a 'wild goose chase' for a bug in that "impossible-to-break, so-very-simple" piece of code that uses pre-/post-'s, remember this post. :)(just thought I'd check it out - turns out I was right :P)

reply to egonfreeman at gmail dot com04-Apr-2007 07:45 the second example u mentioned as follow:=====================================$n = 3;$n * $n++from 3 * 3 into 3 * 4. Post- operations operate on a variable after it has been 'checked', but it doesn't necessarily state that it should happen AFTER an evaluation is over (on the contrary, as a matter of fact).===========================================everything works correctly but one sentence should be modified:"from 3 * 3 into 3 * 4"  should be "from 3 * 3 into 4 * 3"best regards~ :)

Regarding the ternary operator, I would rather say that the best option is to enclose all the expression in parantheses, to avoid errors and improve clarity:<?php   print ( $a > 1 ? "many" : "just one" );?>PS: for php, C++, and any other language that has it.

The short-circuiting IS a feature. It is also available in C, so I suppose the developers won?t remove it in future PHP versions.It is rather nice to write:$file=fopen("foo","r") or die("Error!");Greets,Oliver

To jvm at jvmyers dot com:Your first two if statements just check if there's anything in the string, if you wish to actually execute the code in your string you need eval().

An easy fix (although intuitively tough to do...) is to reverse the comparison.if (5 == $a) {}If you forget the second '=', you'll get a parse error for trying to assign a value to a non-variable.

Regarding 12345alex at gmx dot net's example:I think you miss the identical equal documentation line from: http://php.net/manual/en/language.operators.comparison.php $a == $b     Equal     TRUE if $a is equal to $b after type juggling.$a === $b     Identical     TRUE if $a is equal to $b, and they are of the same type. Try: print array() === NULL ? "True" : "False";Check this:var_dump(is_null(array()));

evaluation order of subexpressions is not strictly defined for all operators<?phpfunction a() {echo 'a';}function b() {echo 'b';}a() == b(); // outputs "ab", ie evaluates left-to-right$a = 3;var_dump( $a == $a = 4 ); // outputs bool(true), ie evaluates right-to-left?>this is not a bug: "we [php developers] make no guarantee about the order of evaluation".See https://bugs.php.net/bug.php?id=61188