Question

How are callbacks written in PHP?

Answer 1

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.