User Define Function in PHP

Function is a collection of logically related statements that are used to perform specific task. 
Function is a useful concept in PHP Script.
When it is required to perform specific task repeatedly in your script you can define a function for that task so there is no need to write same block of code again and again in your script.Thus it will reduce length of PHP Script. 
Hence there is no repetition of same statement again and again in the script it is easy for the user to understand and solve the error.

How to define Function

In PHP there is no need to declare user define function like C and C++.
Function can be defined anywhere in the script and it can be called from anywhere in the script.

Syntax
function FunctionName(ArgumentList)
{
    Function Body (Group of Statements)
}

Argument List is a collection of variables seperated by commas.If function does not pass any arguments then parenthesis are left blank.
Once function is defined inside PHP script you can call it from anywhere in the script any number of times.

     
Syntax
FunctionName(Argument)

Now consider simple example using user define function that display welcome meddage in PHP script.

Example
<?php
//Function Definition
function Message()
{
echo "Welcome To the World of PHP";
}
//Function Call
Message(); 
?>
Output
Welcome TO the World of PHP
Function with Arguments

Sometimes it is required to pass different input values to the function so function can work with that values.This can be done by passing arguments to the function.
Consider Following Example which accept two numbers as an argument and calculate sum of them.

Example
<?php
//Function Declaration
function sum($a,$b)
{
    $c=$a + $b;
    echo "Sum=".$c;
}
//Function Call
sum(3,4);
//Function Call
sum(5,4);
?>
Output
Sum = 7
Sum = 9
Function with Return value

Sometimes it is required to return value from the function to the point from which the function is called. In PHP function can return a single value.
Consider following example which accept two numbers as an argument and return sum of them.

Example
<?php
//Function Declaration
function sum($a,$b)
{
            $c=$a + $b;
            return $c;
}
$a = 4;
$b = 5;
//Function Call
echo "sum of $a and $b is ".sum($a,$b);
$a = 5;
$b = 3;
//Function Call
echo "sum of $a and $b is ".sum($a,$b);
?>
Output
Sum of 4 and 5 is  9
Sum of 5 and 3 is 8
Function with Default Argument

The process of assigning value to the argument at the time of defining function is known as default argument function.
Hence value is assigned to the argument at the time of defining function there is no need to pass its value at the time of calling function.It will automatically consider default value for that argument.
However if you pass value for the default argument at the time of calling the function then it will overwrite the default value of the argument.

Syntax
function FunctionName(Argument1, Argument2=DefaultValue)
{
     Function Body (Group of Statement)
}
Note: Default argument must be specified after all non default arguments.

Consider following example which calculate area of circle using default argument.

Example
<?php
//Function Declaration
function Area($r, $pi=3.14)
{
$a = $pi * $r;
return $a;
}
//Function Call
$r=3;
echo "Area of Circle with Radius $r is".Area($r);
?>
Output
Area of Circle with Radius 3 is 28.26
Function with Variable Length Argument

In PHP you can pass variable number of arguments to the function without defining it in function definition.
It is useful in the situations where you dont know in advance that how much argument will be passed to the function. 
In order to specify Variable length arguments leave the parenthesis blank at the time of defining function.
In PHP following built in functions are used to support variable number of arguments.

Function Purpose
func_num_args()
It is used to find number of arguments passed to the function.
It returns an integer number which contains number of arguments passed to the function.
func_get_args(position)
It is used to find the value of the argument from specific position that is supplied as an argument.
Position of the first argument is 0 and so on. 
func_get_args()
It is used to find the values of all the arguments passed to the function.
It returns an array which contains list of all arguments.

Consider following example which will concate all the arguments that are passed to the function.

Example
<?php
//Function Definition
function Concate()
{
$n = func_num_args();
echo "$n arguments passed to the function<br/>";
$m = "";
$arg = func_get_args();
foreach ($arg as $value)
{
     $m = $m.$value;
}
return $m;
}
//Function Call
echo concate ("PHP", "Server", "Side","Scripting","Language");
?>
Output
5 arguments passed to the function
PHP Server Side Scripting Language

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