Programming PHP, 4th Edition

Chapter 3. Functions

A function is a named block of code that performs a specific task, possibly acting upon a set of values given to it, aka parameters, and possibly returning a single value or set of values in an array. Functions save on compile time—no matter how many times you call them, functions are compiled only once for the page. They also improve reliability by allowing you to fix any bugs in one place rather than everywhere you perform a task, and they improve readability by isolating code that performs specific tasks.

This chapter introduces the syntax of function calls and function definitions and discusses how to manage variables in functions and pass values to functions (including pass-by-value and pass-by-reference). It also covers variable functions and anonymous functions.

Calling a Function

Functions in a PHP program can be built in (or, by being in an extension, effectively built in) or user-defined. Regardless of their source, all functions are evaluated in the same way:

$someValue = function_name( [ parameter, ... ] );

The number of parameters a function requires differs from function to function (and, as we’ll see later, may even vary for the same function). The parameters supplied to the function may be any valid expression and must be in the specific order expected by the function. If the parameters are given out of order, the function may still run by a fluke, but it’s basically a case of “garbage in = garbage out.” A function’s documentation will tell ...