Programming PHP

Chapter 3 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, or parameters, and possibly returning a single value.
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 fixany 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 syntaxof 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 either 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:
$some_value = 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 should be in the specific order
expected by the function. A function’s documentation will tell you what parameters
the function expects and what values you can expect to be returned.
Here are some examples of functions:
// strlen( ) is a built-in function that returns the length of a string
$length = strlen("PHP"); // $length is now 3
// sin() and asin( ) are the sine and arcsine math functions
$result = sin(asin(1)); // $result is the sine of arcsin(1), or 1.0
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