About user notes
PHP versions documented in this manual
About the manual
PHP Manual

## 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 = FALSE ] )

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.

In addition the & (ampersand) symbol prepended to a function parameter allows the parameter to be passed by reference, as seen below:

           int preg_match ( string $pattern , string $subject [, array &$matches
          [, int $flags = 0 [, int $offset = 0 ]]] )

In this example, we can see the third optional parameter `&$matches` will be passed as reference.

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.

About user notes
PHP versions documented in this manual
About the manual
PHP Manual