How do you Implement Callbacks in PHP?

Callback Implementation in PHP

In PHP, callbacks, also referred to as "callable" values, enable the referencing of functions or class methods for future invocation. They serve as a fundamental concept in functional programming and have been supported since PHP 4.

Callback Syntax

Callbacks can be expressed in various forms:

• String: The callback value is a string representing a function name.
• Array: An array specifying the class name and the static method name.
• Object and Method: An array containing an object instance and a method name.
• Callable String: A string representing a class name followed by a method name using PHP 5.2.3 syntax (however, this syntax can't be directly called).

Callback Invocation

Callbacks can be safely invoked using the is_callable() function to verify their validity. If the callback is callable, it can be invoked using:

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

Direct Invocation

Modern PHP versions support directly invoking the first three callback formats (string, array, and object with method). One can also use call_user_func and call_user_func_array to call all callback formats.

Additional Notes

• Namespaced functions/classes require fully-qualified names in the callback string.
• call_user_func doesn't support passing non-objects by reference.
• Objects with an __invoke() method can be used as callbacks.
• create_function() has been deprecated and should be replaced with anonymous functions.

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