Which function is used to start a session in PHP?

The function used to start a session in PHP is session_start(). When a PHP script calls this function, it initializes a session or resumes the current one based on a session identifier passed via a request, typically using a cookie. This is fundamental for maintaining state across multiple page requests, as PHP is stateless by default. To work with session variables, which are stored on the server and are used to persist user-specific data, session_start() must be called before any output is sent to the browser. This makes it a pivotal function in handling user sessions, allowing developers to manage user data effectively throughout their browsing session.

The other options do not exist as valid PHP functions for starting a session. For example, session_init(), session_open(), and session_create() are not defined in the PHP manual and will lead to errors if used in code. Proper understanding of session management in PHP is essential for web development as it directly impacts user experience and data handling.