PHP sessions are used to store user-specific data on the server between HTTP requests. When a user visits a website, PHP creates a unique session ID and a new session is started. The session ID is then sent to the user’s browser in the form of a cookie, and is used to identify the user on subsequent requests.

To start a session in PHP, you can use the session_start() function, which will check if a session already exists and create one if it doesn’t. Once a session is started, you can store and retrieve data in the $_SESSION superglobal array. For example, you can store a value in the session like this:

session_start();
$_SESSION["username"] = "johndoe";

And retrieve it later like this:

session_start();
echo $_SESSION["username"]; // Outputs "johndoe"

You can also use the session_destroy() function to end a session and delete all session data.

It’s important to note that, by default, PHP stores session data on the server in a temporary file, and this may not be secure. You should consider using a more secure storage method, such as storing session data in a database.