What are the potential security risks of using $_SESSION variables to store user data in PHP?

Using $_SESSION variables to store user data in PHP can pose security risks if the data is not properly sanitized or validated. This can lead to vulnerabilities such as session hijacking, session fixation, or data tampering. To mitigate these risks, it is important to always sanitize and validate user input before storing it in $_SESSION variables.

// Sanitize and validate user input before storing it in $_SESSION variable
$_SESSION['username'] = filter_var($_POST['username'], FILTER_SANITIZE_STRING);