Function In PHP

                     PHP Function

A function is a piece of code which takes one more input in the form of parameter and does some processing and returns a value.

There are two types functions:

1- Pre-defined or Library Functions :
    there are already more than 1000 of built-in library functions created for different area and you just need to call them according to your requirement.
         EX- isset(), unset(), strlen(), strrev(), fopen(), fread() etc are Pre-defined or Library Function.


2- User Defined Functions :
There are two parts which should be clear to you:
Creating a PHP Function
Calling a PHP Function

Creating PHP Function And Calling Function:

Its very easy to create your own PHP function. Suppose you want to create a PHP function which will simply write a simple message on your browser when you will call it. Following example creates a function called writeMessage() and then calls it just after creating it.
Note that while creating a function its name should start with keyword function and all the PHP code should be put inside { and } braces as shown in the following example below:

<?php
// Defining a PHP Function 
function writeMessage()
{
  echo "You are really a nice person, Have a nice time!";
}
// Calling a PHP Function 
writeMessage();
?>

This will display following result:
You are really a nice person, Have a nice time!





PHP Functions with Parameters:

PHP gives you option to pass your parameters inside a function. You can pass as many as parameters your like. These parameters work like variables inside your function. Following example takes two integer parameters and add them together and then print them.

<?php
function addFunction($num1, $num2)
{
  $sum = $num1 + $num2;
  echo "Sum of the two numbers is : $sum";
}
addFunction(10, 20);
?>

This will display following result:
Sum of the two numbers is : 30



Passing Arguments by Reference:

It is possible to pass arguments to functions by reference. This means that a reference to the variable is manipulated by the function rather than a copy of the variable's value.
Any changes made to an argument in these cases will change the value of the original variable. You can pass an argument by reference by adding an ampersand to the variable name in either the function call or the function definition.

Following example depicts both the cases.

<?php
function addFive($num)
{
   $num += 5;
}
function addSix(&$num)
{
   $num += 6;
}
$orignum = 10;
addFive( &$orignum );
echo "Original Value is $orignum<br />";
addSix( $orignum );
echo "Original Value is $orignum<br />";
?>

This will display following result:
Original Value is 15
Original Value is 21



PHP Functions retruning value:

A function can return a value using the return statement in conjunction with a value or object. return stops the execution of the function and sends the value back to the calling code.
You can return more than one value from a function using return array(1,2,3,4).
Following example takes two integer parameters and add them together and then returns their sum to the calling program. Note that return keyword is used to return a value from a function.

<?php
function addFunction($num1, $num2)
{
  $sum = $num1 + $num2;
  return $sum;
}
$return_value = addFunction(10, 20);
echo "Returned value from the function : $return_value
?>

This will display following result:
Returned value from the function : 30



Dynamic Function Calls:

It is possible to assign function names as strings to variables and then treat these variables exactly as you would the function name itself. Following example depicts this behaviour.

<?php
function sayHello()
{
   echo "Hello<br />";
}
$function_holder = "sayHello";
$function_holder();
?>

This will display following result:
Hello