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, PHP 7)
strlen -- Get string length

Description
strlen ( string $string ) : int

Returns the length of given string.

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

      function name    ( parameter type   parameter name ) : returned type

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

      in_array ( mixed $needle, array $haystack , bool $strict = false ) : bool

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 have default values; if the default value is unknown, it is shown as ?. 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:

       preg_match ( string $pattern , string $subject , array &$matches = null,
       int $flags = 0 , int $offset = 0 ) : int|false

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, PHP 7)

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