How can the use of deprecated HTTP_* variables in PHP impact session management and variable access?

Deprecated HTTP_* variables in PHP can impact session management and variable access because they are not reliable and may not always contain the expected values. To avoid potential issues, it is recommended to use the $_SERVER superglobal array instead to access HTTP headers and other server variables.

// Example of accessing HTTP headers using $_SERVER superglobal
$user_agent = $_SERVER['HTTP_USER_AGENT'];