Whilst it was correct 11 years ago, the statement of Dan D is not so correct any moreю Anonymous functions are now objects of a class Closure and are safely collected by garbage collector.

Beware when using anonymous functions in PHP as you would in languages like Python, Ruby, Lisp or Javascript.  As was stated previously, the allocated memory is never released; they are not objects in PHP -- they are just dynamically named global functions -- so they don't have scope and are not subject to garbage collection.

So, if you're developing anything remotely reusable (OO or otherwise), I would avoid them like the plague.  They're slow, inefficient and there's no telling if your implementation will end up in a large loop.  Mine ended up in an iteration over ~1 million records and quickly exhasted my 500MB-per-process limit.

In regards to the recursion issue by info at adaniels dot nl

Anon function recursion by referencing the function variable in the correct scope.
<?php
$fn2 = create_function('$a', 'echo $a; if ($a < 10) call_user_func($GLOBALS["fn2"], ++$a);');
$fn2(1);
?>

Try this to boost performance of your scripts (increase maxCacheSize):

<?php
runkit_function_copy('create_function', 'create_function_native');
runkit_function_redefine('create_function', '$arg,$body', 'return __create_function($arg,$body);');

function __create_function($arg, $body) {
    static $cache = array();
    static $maxCacheSize = 64;
    static $sorter;

    if ($sorter === NULL) {
        $sorter = function($a, $b) {
            if ($a->hits == $b->hits) {
                return 0;
            }

            return ($a->hits < $b->hits) ? 1 : -1;
        };
    }

    $crc = crc32($arg . "\\x00" . $body);

    if (isset($cache[$crc])) {
        ++$cache[$crc][1];
        return $cache[$crc][0];
    }

    if (sizeof($cache) >= $maxCacheSize) {
        uasort($cache, $sorter);
        array_pop($cache);
    }

    $cache[$crc] = array($cb = eval('return function('.$arg.'){'.$body.'};'), 0);
    return $cb;
}
?>

In the process of migrating a PHP4 codebase to PHP5, I ran into a peculiar problem. In the library, every class was derived from a generic class called 'class_container'. 'class_container' contained an array called runtime_functions and a method called class_function that was as follows:

<?php
function class_function($name,$params,$code) {

  $this->runtime_functions[$name] = create_function($params,$code);

}
?>

In a subclass of class_container, there was a function that utilized class_function() to store some custom lambda functions that were self-referential:

<?php
function myfunc($name,$code) {

  $this->class_function($name,'$theobj','$this=&$theobj;'.$code);

}
?>

In PHP4, this worked just fine. The idea was to write blocks of code at the subclass level, such as "echo $this->id;", then simply $MYOBJ->myfunc("go","echo $this->id;"); and later call it like $MYOBJ->runtime_functions["go"]();

It essentially worked exactly like binding anonymous functions to objects in Javascript.

Note how the "$this" keyword had to be manually redefined for the $code block to work.

In PHP5, however, you can't redeclare $this without getting a fatal error, so the code had to be updated to:

<?php
function myfunc($name,$code) {

  $this->class_function($name,'$this',$code);

}
?>

Apparently create_function() allows you to set $this via a function argument, allowing you to bind anonymous functions to instantiated objects. Thought it might be useful to somebody.

Note that using __FUNCTION__ in a an anonymous function, will always result '__lambda_func'.

<?php
    $fn = create_function('', 'echo __FUNCTION__;');
    $fn();
    // Result: __lambda_func
    echo $fn;
    // Result: ºlambda_2 (the actual first character cannot be displayed)
?>

This means that a anonymous function can't be used recursively. The following code (recursively counting to 10) results in an error:
<?php
    $fn2 = create_function('$a', 'echo $a; if ($a < 10) call_user_func(__FUNCTION__, $a++);');
    $fn2(1);
    // Warning: call_user_func(__lambda_func) [function.call-user-func]: First argument is expected to be a valid callback in T:/test/test.php(21) : runtime-created function on line 1
?>

The following function is very useful for creating an alias of a user function. 
For built-in functions, it is less useful because default values are not available, so function aliases for built-in functions must have all parameters supplied, whether optional or not.

<?php
function create_function_alias($function_name, $alias_name)
{
    if(function_exists($alias_name))
        return false;
    $rf = new ReflectionFunction($function_name);
    $fproto = $alias_name.'(';
    $fcall = $function_name.'(';
    $need_comma = false;
    
    foreach($rf->getParameters() as $param)
    {
        if($need_comma)
        {
            $fproto .= ',';
            $fcall .= ',';
        }

        $fproto .= '$'.$param->getName();
        $fcall .= '$'.$param->getName();

        if($param->isOptional() && $param->isDefaultValueAvailable())
        {
            $val = $param->getDefaultValue();
            if(is_string($val))
                $val = "'$val'";
            $fproto .= ' = '.$val;
        }
        $need_comma = true;
    }
    $fproto .= ')';
    $fcall .= ')';

    $f = "function $fproto".PHP_EOL;
    $f .= '{return '.$fcall.';}';

    eval($f);
    return true;
}
?>

For who *really* needs the create_function() on php8 (because of legacy code that cannot be changed easily) there is this: "composer require lombax85/create_function".

Here has been some discussion about the "memory leak" create_function() can create.

What create_function() actually does, is creating an ordinary function with name chr(0).lambda_n where n is some number:

<?php
$f = create_function('', 'return 1;');

function lambda_1() { return 2; }

$g = "lambda_1";
echo $g(); // outputs: 2

$h = chr(0)."lambda_1";
echo $h(); // outputs: 1
?>

Here's how to call a runtime-created function from another runtime-created function:
<?php
        $get_func = create_function('$func', 'return substr($func,1);');
        $get_value = create_function('$index','return pow($index,$index);');
        $another_func = create_function('$a', '$func="\x00"."'.$get_func($get_value).'";return $func($a);');
        echo $another_func(2); # result is 4
?>

In reply to info at adaniels dot nl:

You may not be able to use __FUNCTION__ in a lambda (thanks for pointing it out; I was having that problem just now), but you can use $GLOBALS to work around it if you're assigning the function to a variable. I reimplemented array_walk_recursive() in PHP4 like this:

<?php
$array_walk_recursive = create_function('&$array, $callback',
    'foreach($array as $element) {
        if(is_array($element)) {
            $funky = $GLOBALS["array_walk_recursive"];
            $funky($element, $callback);
        }
        else {
            $callback($element);
        }
    }');
?>

Beware! This is merely a convenience function that generates a unique name for a regular function. It is *not* a closure or even an anonymous function. It is just a regular function that gets named for you.

Functions created by create_function() cannot return a value by reference.  The function below creates a function that can.  The arguments are the same as create_function().  Note that these arguments are passed, unmodified, to eval(), so be sure that data passed in is sanitized.

<?php
/** 
 * create_ref_function
 * Create an anonymous (lambda-style) function
 * which returns a reference
 * see http://php.net/create_function
 */
function
create_ref_function( $args, $code )
{
    static $n = 0;

    $functionName = sprintf('ref_lambda_%d',++$n);
    
    $declaration = sprintf('function &%s(%s) {%s}',$functionName,$args,$body);
    
    eval($declaration);
    
    return $functionName;
}
?>

neo at gothic-chat d0t de wrote :
Beware of memory-leaks, the garbage-collection seems to 'oversee' dynamically created functions!

Not really...

In fact, PHP can not "unassign" functions. So if you create a function, it won't be deleted until the end of the script, even if you unset the variable containing its name.

If you need to change a part of a function everytime you run a loop, think of a way to make a more general function or try using eval :) (functions are made to be re-used. If you need to run your own piece of code once, eval is much better).

[EDIT by danbrown AT php DOT net: Combined user-corrected post with previous (incorrect) post.]

You can't refer to a class variable from an anonymous function inside a class method using $this.  Anonymous functions don't inherit the method scope.  You'll have to do this:

<?php
class AnyClass {
 
 var $classVar = 'some regular expression pattern';

 function classMethod() {

   $_anonymFunc = create_function( '$arg1, $arg2', 'if ( eregi($arg2, $arg1) ) { return true; } else { return false; } ' );

   $willWork = $_anonymFunc('some string', $classVar);
 
 }

}
?>

Best wapper:

<?php

function create_lambda($args, $code) {
    static $func;
    if (!isset($func[$args][$code])) {
        $func[$args][$code] = create_function($args, $code);
    }
    return $func[$args][$code];
}

In response to kkaiser at revolution-records dot net's note, even tho PHP will allow you to use 
<?
$myfunc = create_function('$this', $code);
?>
You can NOT use a reference to "$this" inside of the anonymous function, as PHP will complain that you are using a reference to "$this" in a non-object context.

Currently, I have not found a work-around for this...

Just a little toy I thought up, I would like to share. Creates an anonymous function, which let you use a class  as a function.

In php 5.3 there is support for real functors  (trough __invoke):

<?php
function createFunctor($className){
        $content = "
                static \$class;
                if(!\$class){
                        \$class = new $className;
                }
                return \$class->run(\$args);
        ";
        $f = create_function('$args', $content);
        return $f;

}
class test {
        public function run($args){
                print $args;
        }
}
$test = createFunctor('test');
$test('hello world');
?>

$f = create_function('','echo "function defined by create_function";');
$f();

result:
function defined by create_function

You may define no return in function body while you are using create_function.

If you were checking to see if a function is made properly, this would be a better way of checking:

<?php
$fnc = @create_function('$arg1,$arg2,$arg3', 'return true;');
# make that function whatever you want
if (empty($fnc)) {
  die('Could not create function $fnc.');
}

# although, the follow will NOT work
if (empty(create_function('$arg', 'return $arg;'))) {
  die('Could not create anonymous function.');
}
# you would get an error regarding not being able to use a
# return value in writeable context (i.e. a return value is 
# a const in C, and the function empty() doesn't use a
# const void* parameter
?>

Beware of memory-leaks, the garbage-collection seems to 'oversee' dynamically created functions!

I used a function like this to replace special characters in links with their htmlentities:
<?php
$text = preg_replace_callback (
    "/(<(frame src|a href|form action)=\")([^\"]+)(\"[^>]*>)/i",
    create_function (
        '$matches',
        'return $matches[1] . htmlentities ($matches[3]) . $matches[4];'
    ),
    $text);
?>

After 1000 calls, the process used about 5MB more than before. In my situation this boosted up the memory-size of one PHP-process up to over 100MB!

In such cases, better store the function in a global variable.

Create_function enables the ability to change the scope of functions.  You might have a class where it needs to define a GLOBAL function.  This is possible, like:

<?php
        class blah {
                function blah() {
                        $z=create_function('$arg1string','return "function-z-".$arg1string;');
                        $GLOBALS['z']=$z;
                }
        }
        $blah_object=new blah;

        $result=$GLOBALS['z']('Argument 1 String');
        echo $result;
?>

Making a function escape it's defined scope can be useful in many situations.