How do I implement a callback in PHP?

How are callbacks written in PHP?

The manual uses the terms "callback" and "callable" interchangeably, however, "callback" traditionally refers to a string or array value that acts like a function pointer, referencing a function or class method for future invocation. This has allowed some elements of functional programming since PHP 4. The flavors are:

$cb1 = 'someGlobalFunction';
$cb2 = ['ClassName', 'someStaticMethod'];
$cb3 = [$object, 'somePublicMethod'];

// this syntax is callable since PHP 5.2.3 but a string containing it
// cannot be called directly
$cb2 = 'ClassName::someStaticMethod';
$cb2(); // fatal error

// legacy syntax for PHP 4
$cb3 = array(&$object, 'somePublicMethod');

This is a safe way to use callable values in general:

if (is_callable($cb2)) {
    // Autoloading will be invoked to load the class "ClassName" if it's not
    // yet defined, and PHP will check that the class has a method
    // "someStaticMethod". Note that is_callable() will NOT verify that the
    // method can safely be executed in static context.

    $returnValue = call_user_func($cb2, $arg1, $arg2);
}

Modern PHP versions allow the first three formats above to be invoked directly as $cb(). call_user_func and call_user_func_array support all the above.

See: http://php.net/manual/en/language.types.callable.php

Notes/Caveats:

1. If the function/class is namespaced, the string must contain the fully-qualified name. E.g. ['Vendor\Package\Foo', 'method']
2. call_user_func does not support passing non-objects by reference, so you can either use call_user_func_array or, in later PHP versions, save the callback to a var and use the direct syntax: $cb();
3. Objects with an __invoke() method (including anonymous functions) fall under the category "callable" and can be used the same way, but I personally don't associate these with the legacy "callback" term.
4. The legacy create_function() creates a global function and returns its name. It's a wrapper for eval() and anonymous functions should be used instead.

With PHP 5.3, you can now do this:

function doIt($callback) { $callback(); }

doIt(function() {
    // this will be done
});

Finally a nice way to do it. A great addition to PHP, because callbacks are awesome.

Implementation of a callback is done like so

// This function uses a callback function. 
function doIt($callback) 
{ 
    $data = "this is my data";
    $callback($data); 
} 


// This is a sample callback function for doIt(). 
function myCallback($data) 
{ 
    print 'Data is: ' .  $data .  "\n"; 
} 


// Call doIt() and pass our sample callback function's name. 
doIt('myCallback');

Displays: Data is: this is my data

One nifty trick that I've recently found is to use PHP's create_function() to create an anonymous/lambda function for one-shot use. It's useful for PHP functions like array_map(), preg_replace_callback(), or usort() that use callbacks for custom processing. It looks pretty much like it does an eval() under the covers, but it's still a nice functional-style way to use PHP.

well... with 5.3 on the horizon, all will be better, because with 5.3, we'll get closures and with them anonymous functions

http://wiki.php.net/rfc/closures

You will want to verify whatever your calling is valid. For example, in the case of a specific function, you will want to check and see if the function exists:

function doIt($callback) {
    if(function_exists($callback)) {
        $callback();
	} else {
        // some error handling
    }
}

create_function did not work for me inside a class. I had to use call_user_func.

<?php

class Dispatcher {
    //Added explicit callback declaration.
    var $callback;

    public function Dispatcher( $callback ){
         $this->callback = $callback;
    }

    public function asynchronous_method(){
       //do asynch stuff, like fwrite...then, fire callback.
       if ( isset( $this->callback ) ) {
			if (function_exists( $this->callback )) call_user_func( $this->callback, "File done!" );
		}
    }

}

Then, to use:

<?php 
include_once('Dispatcher.php');
$d = new Dispatcher( 'do_callback' );
$d->asynchronous_method();

function do_callback( $data ){
   print 'Data is: ' .  $data .  "\n";
}
?>

[Edit] Added a missing parenthesis. Also, added the callback declaration, I prefer it that way.

I cringe every time I use create_function() in php.

Parameters are a coma separated string, the whole function body in a string... Argh... I think they could not have made it uglier even if they tried.

Unfortunately, it is the only choice when creating a named function is not worth the trouble.

For those who don't care about breaking compatibility with PHP < 5.4, I'd suggest using type hinting to make a cleaner implementation.

function call_with_hello_and_append_world( callable $callback )
{
     // No need to check $closure because of the type hint
     return $callback( "hello" )."world";
}

function append_space( $string )
{
     return $string." ";
}

$output1 = call_with_hello_and_append_world( function( $string ) { return $string." "; } );
var_dump( $output1 ); // string(11) "hello world"

$output2 = call_with_hello_and_append_world( "append_space" );
var_dump( $output2 ); // string(11) "hello world"

$old_lambda = create_function( '$string', 'return $string." ";' );
$output3 = call_with_hello_and_append_world( $old_lambda );
var_dump( $output3 ); // string(11) "hello world"