session_start()
Initialize session data
Syntax : bool session_start ( void )
session_start() creates a session or resumes the current one based on the current session id that’s being passed via a request, such as GET, POST, or a cookie.
If you want to use a named session, you must call session_name() before calling session_start().
session_start() will register internal output handler for URL rewriting when trans-sid is enabled. If a user uses ob_gzhandler or like with ob_start(), the order of output handler is important for proper output. For example, user must register ob_gzhandler before session start.
Parameters :
None

Return Values :
This function always returns TRUE.
Examples :

<?php
		session_start();
		if(isset($_SESSION['views']))
    			$_SESSION['views'] = $_SESSION['views']+ 1;
		else
    			$_SESSION['views'] = 1;
		echo "views = ". $_SESSION['views'];
?>

DISPLAY : views = 1

Cleaning and Destroying your Session

PHP Code:

<?php
		session_start();
		if(isset($_SESSION['cart']))
    			unset($_SESSION['cart']);
?>

You can also completely destroy the session entirely by calling the session_destroy function.

<?php
		session_start();
		session_destroy();
?>