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In computer programming, a callback is a function that is stored as data (a reference) and designed to be called by another function – often back to the original abstraction layer. A function that accepts a callback parameter may be designed to call back before returning to its caller which is known as synchronous or blocking .
PHP has a direct module interface called SAPI for different web servers; [273] in case of PHP 5 and Apache 2.0 on Windows, it is provided in form of a DLL file called php5apache2.dll, [274] which is a module that, among other functions, provides an interface between PHP and the web server, implemented in a form that the server understands. This ...
Events can be implemented through various mechanisms such as callbacks, message objects, signals, or interrupts, and events themselves are distinct from the implementation mechanisms used. Event propagation models, such as bubbling, capturing, and pub/sub, define how events are distributed and handled within a system.
The term closure is often used as a synonym for anonymous function, though strictly, an anonymous function is a function literal without a name, while a closure is an instance of a function, a value, whose non-local variables have been bound either to values or to storage locations (depending on the language; see the lexical environment section below).
A typical use of a function object is in writing callback functions. A callback in procedural languages, such as C, may be performed by using function pointers. [2] However it can be difficult or awkward to pass a state into or out of the callback function. This restriction also inhibits more dynamic behavior of the function.
In an event-driven application, there is generally an event loop that listens for events and then triggers a callback function when one of those events is detected. Event-driven programs can be written in any programming language, although the task is easier in languages that provide high-level abstractions.
Function pointers allow different code to be executed at runtime. They can also be passed to a function to enable callbacks. Function pointers are supported by third-generation programming languages (such as PL/I, COBOL, Fortran, [1] dBASE dBL [clarification needed], and C) and object-oriented programming languages (such as C++, C#, and D). [2]
PHP has hundreds of base functions and thousands more from extensions. Prior to PHP version 5.3.0, functions are not first-class functions and can only be referenced by their name, whereas PHP 5.3.0 introduces closures. [35] User-defined functions can be created at any time and without being prototyped. [35]