PHP Manual: Start new or resume existing session

Description

bool session_start(array $options = [])

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.

Parameters

options

If provided, this is an associative array of options that will override the currently set session configuration directives. The keys should not include the session. prefix.

In addition to the normal set of configuration directives, a read_and_close option may also be provided. If set to true, this will result in the session being closed immediately after being read, thereby avoiding unnecessary locking if the session data won't be changed.

Return Values

This function returns true if a session was successfully started, otherwise false.

Changelog

Version	Description
7.1.0	session_start now returns `false` and no longer initializes `$_SESSION` when it failed to start the session.

Examples

A basic session example

Example #1 page1.php

<?php
// page1.php

session_start();

echo '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 page2.php

<?php
// page2.php

session_start();

echo 'Welcome to page #2<br />';

echo $_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>';
?>

Providing options to session_start

Example #3 Overriding the cookie lifetime

<?php
// This sends a persistent cookie that lasts a day.
session_start([
    'cookie_lifetime' => 86400,
]);
?>

Example #4 Reading the session and closing it

<?php
// If we know we don't need to change anything in the
// session, we can just read and close rightaway to avoid
// locking the session file and blocking other pages
session_start([
    'cookie_lifetime' => 86400,
    'read_and_close'  => true,
]);

Notes

Note:
To use cookie-based sessions, session_start must be called before outputting anything to the browser.

Note:
Use of zlib.output_compression is recommended instead of ob_gzhandler

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

session_start