Implementing Callbacks in PHP

The term "callback" in PHP encompasses both strings and arrays that operate as function pointers. In PHP 4, the following syntax emerged:

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

Although PHP 5.2.3 introduced callable syntax, strings containing such syntax cannot be directly invoked. Legacy syntax for PHP 4 includes:

• $cb3 = array(&$object, 'somePublicMethod');

The following code snippet demonstrates safe usage of callable values:

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

Modern PHP versions support invoking the first three formats above directly as $cb(). Additionally, call_user_func and call_user_func_array support all the presented formats.

Notes and Caveats:

• Namespaced functions/classes require fully-qualified names, e.g. ['Vendor\Package\Foo', 'method'].
• call_user_func does not support passing non-objects by reference. Use call_user_func_array or $cb(); in later PHP versions.
• Objects with an __invoke() method (including anonymous functions) can be used as callbacks, but are not typically associated with the traditional "callback" term.
• Legacy create_function() creates a global function and returns its name. Use anonymous functions instead.