The session_start() function in PHP creates a session or resumes the current one based on a session identifier passed via a GET or POST request, or passed via a cookie¹. A session is a way to store information (in variables) to be used across multiple pages².

The session_start() function must be the very first thing in your document, before any HTML tags². It will call the open and read session save handlers, which will either be a built-in save handler provided by default or by PHP extensions, or a custom handler as defined by session_set_save_handler()¹. The read callback will retrieve any existing session data and populate the $_SESSION superglobal array¹.

To use a named session, call session_name() before calling session_start()¹. The name attribute of the session is needed to reference the form data after the form is submitted².

The session_start() function returns true if a session was successfully started, otherwise false¹. If you want to use multiple sessions in the same script, you can use session_id() to change the session identifier before calling session_start()³.

Here is an example of using the session_start() function to create a new PHP session and set some session variables:

You can access the session variables on other pages by calling session_start() again and using the $_SESSION array: