PHP Functions
Learn about functions in PHP and their types.
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Functions in PHP
Functions are the heart of functional programming. Functions are relationships between inputs and outputs. The arity of a function is the number of arguments a function accepts, and the function signature specifies return types and argument types. Lastly, the definition of a function encapsulates the function logic. Here’s a mathematically defined function:
$f(x) = x + 1$
It has an arity of one, and it increments the value of the parameter $x$ by 1. The parameter $x$ is effectively a placeholder for a general transformation given in the function definition using the addition operation: $x + 1$. Different results are obtained for different values of $x$ because the function offers localized computation. Each result is, therefore, unique to each input. For example:
$f(2) = 3$
$f(3) = 4$
Functions in PHP take on many forms. They’re just as flexible as variables and have the following characteristics:

The
function
keyword. 
Two sets of parentheses:
 Round brackets
()
for the arguments.  Curly braces
{}
for the function definition.
 Round brackets

A return type (as of PHP version 7.0).
Types of functions in PHP
The different types of functions in PHP are the following:
 Named functions
 Anonymous functions
 Closures
 Methods
Named functions
Named functions in PHP, like the one below, are identical (apart from a few syntactic differences) to those defined in other programming languages. They typically have an arbitrary name sandwiched between a function keyword and an argument list followed by the function definition, that contains state transformation expressions.
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