How Can I Use Callbacks Effectively in PHP?

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:

<code class="php">if (is_callable($cb2)) {
    $returnValue = call_user_func($cb2, $arg1, $arg2);
}</code>

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. ['VendorPackageFoo', '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.

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