PHPSESSID is a session cookie that is used to identify a user's session on a website. It is commonly used in PHP-based web applications to store a unique identifier for a user's session on the server. When a user visits a website that uses PHP sessions, a PHPSESSID cookie is sent to the user's browser and is stored on the user's computer. The server uses this cookie to identify the user's session and retrieve the appropriate data for that session. The PHPSESSID cookie is typically set to expire when the user closes their web browser.