How to read a function definition (prototype)

Each function in the manual is documented for quick reference. Knowing how to read and understand the text will make learning PHP much easier. Rather than relying on examples or cut/paste, everyone should know how to read function definitions (prototypes). Let's begin:

Note: Prerequisite: Basic understanding of types

Although PHP is a loosely typed language, it's important to have a basic understanding of types as they have important meaning.

Function definitions tell us what type of value is returned. Let's use the definition for strlen() as our first example:

strlen

(PHP 4, PHP 5)
strlen -- Get string length

Description
int strlen ( string $string )

Returns the length of given string.

Explanation of a function definition

Part	Description
strlen	The function name.
(PHP 4, PHP 5)	strlen() has been around in all versions of PHP 4 and PHP 5
int	Type of value this function returns, which is an integer (i.e. the length of a string is measured in numbers).
( string $string )	The first (and in this case the only) parameter/argument for this function is named string, and it's a string.

We could rewrite the above function definition in a generic way:

      returned type    function name    ( parameter type   parameter name )

Many functions take on multiple parameters, such as in_array(). Its prototype is as follows:

      bool in_array ( mixed $needle, array $haystack [, bool $strict])

What does this mean? in_array() returns a boolean value, TRUE on success (if the needle was found in the haystack) or FALSE on failure (if the needle was not found in the haystack). The first parameter is named needle and it can be of many different types, so we call it "mixed". This mixed needle (what we're looking for) can be either a scalar value (string, integer, or float), or an array. haystack (the array we're searching in) is the second parameter. The third optional parameter is named strict. All optional parameters are seen in [ brackets ]. The manual states that the strict parameter defaults to boolean FALSE. See the manual page on each function for details on how they work.

There are also functions with more complex PHP version information. Take html_entity_decode() as an example:

(PHP 4 >= 4.3.0, PHP 5)

This means that this function has only been available in a released version since PHP 4.3.0.