Functions in PHP - how to use with complete example

PHP function is a piece of code that can be reused many times. PHP functions are similar to other programming languages.

More than thousands pr-define functions are available in PHP. There are many advantage of PHP Functions like Code Reusability, Easy to understand, Less Code etc. A user-defined function declaration starts with the word Function. 

A function name can start with a letter or underscore (not a number). The best practise is given, the function a name that reflects what the function does.

PHP Functions:

function <function_name>(){
....
}

In PHP, we can define Simple function, Parameterized function and Recursive function also.

Let's take a exmpale for all:

Simple function:

In the example, we create a function named "myFunction()". The opening curly brace  { indicates the beginning of the function code and the closing curly brace } indicates the end of the function. Function have some data for print:

function myFunction() {
    echo "Hello world!";
}
myFunction(); // call the function

Output of the function:

Hello world!

Parameterized function:

In the example, You can pass any number of parameters inside a function. These passed parameters act as variables inside your function.. Function have some data for print:

function add($x, $y) {  
     $sum = $x + $y;  
     echo "Sum of two numbers is = $sum <br><br>";  
}
add(12,20); // call the function

Output of the function:

Sum of two numbers is = 32

Recursive function:

PHP also supports recursive function call like C/C++. In such case, we call current function within function. It is also known as recursion. Let's take a exmpale for factorial of a number :

function factorial($n)    
{    
    if ($n < 0)    
        return -1; /*Wrong value*/    
    if ($n == 0)    
        return 1; /*Terminate condition*/    
    return ($n * factorial ($n -1));  
}
factorial(5); // call the function

Output of the function:

120

Call By Reference function:

In call by reference, actual value is modified if it is modified inside the function. In such case, you need to use & (ampersand) symbol with formal arguments. In this example, variable $str is passed to the adder function where it is concatenated with 'Call By Reference' string. Here, printing $str variable results 'This is Call By Reference'.

function reference(&$str2)  
{  
    $str2 .= 'Call By Reference';  
}  
$str = 'This is ';  
reference($str);  // call the function
echo $str;   

Output of the function:

This is Call By Reference