A session in PHP is a way to store data temporarily and make it accessible across multiple pages. Unlike a cookie, the information is not stored on the users computer, but rather on the server. When a session is started with the session_start() function, a temporary file is created that stores various session variables and their values. Session variables are set with the PHP global variable $_SESSION . The $_SESSION variable is an associative array that contains all session variables. To set session variables, you can use the `$_SESSION. The server can then access these global variables until it terminates the session.

To start a new session, the session_start() function is used. It is recommended to put the session_start() function as the first line in your code, even before any HTML tags. To change a session variable, just overwrite it. To remove a session variable, use the unset() function. To completely destroy a session, use the session_destroy() function.

In summary, a session in PHP is a way to store data temporarily and make it accessible across multiple pages. It is created with the session_start() function and session variables are set with the $_SESSION global variable. To change or remove a session variable, you can overwrite it or use the unset() function. To completely destroy a session, use the session_destroy() function.