Start new or resume existing session
Session Functions
PHP Manual


(PHP 4, PHP 5)

session_startStart new or resume existing session


bool session_start ( void )

session_start() creates a session or resumes the current one based on a session identifier passed via a GET or POST request, or passed via a cookie.

When session_start() is called or when a session auto starts, PHP will call the open and read session save handlers. These will either be a built-in save handler provided by default or by PHP extensions (such as SQLite or Memcached); or can be custom handler as defined by session_set_save_handler(). The read callback will retrieve any existing session data (stored in a special serialized format) and will be unserialized and used to automatically populate the $_SESSION superglobal when the read callback returns the saved session data back to PHP session handling.

To use a named session, call session_name() before calling session_start().

When session.use_trans_sid is enabled, the session_start() function will register an internal output handler for URL rewriting.

If a user uses ob_gzhandler or similar with ob_start(), the function order is important for proper output. For example, ob_gzhandler must be registered before starting the session.

Return Values

This function returns TRUE if a session was successfully started, otherwise FALSE.


Version Description
5.3.0 If a session fails to start, then FALSE is returned. Previously TRUE was returned.
4.3.3 As of PHP 4.3.3, calling session_start() after the session was previously started will result in an error of level E_NOTICE. Also, the second session start will simply be ignored.


Example #1 A session example: page1.php

// page1.php


'Welcome to page #1';

$_SESSION['favcolor'] = 'green';
$_SESSION['animal']   = 'cat';
$_SESSION['time']     = time();

// Works if session cookie was accepted
echo '<br /><a href="?page2.php">page 2</a>';

// Or maybe pass along the session id, if needed
echo '<br /><a href="?page2.php?' SID '">page 2</a>';

After viewing page1.php, the second page page2.php will magically contain the session data. Read the session reference for information on propagating session ids as it, for example, explains what the constant SID is all about.

Example #2 A session example: page2.php

// page2.php


'Welcome to page #2<br />';

$_SESSION['favcolor']; // green
echo $_SESSION['animal'];   // cat
echo date('Y m d H:i:s'$_SESSION['time']);

// You may want to use SID here, like we did in page1.php
echo '<br /><a href="?page1.php">page 1</a>';



To use cookie-based sessions, session_start() must be called before outputing anything to the browser.


Use of zlib.output_compression is recommended instead of ob_gzhandler()


This function sends out several HTTP headers depending on the configuration. See session_cache_limiter() to customize these headers.

See Also

Session Functions
PHP Manual