How can PHP developers avoid dependencies on register_globals=on when accessing server variables like HTTP_REFERER?

When register_globals=on is enabled, server variables like HTTP_REFERER are automatically turned into global variables, which can lead to security vulnerabilities and unpredictable behavior. To avoid dependencies on register_globals=on, PHP developers should explicitly access server variables using superglobals like $_SERVER['HTTP_REFERER'] instead.

$referer = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : '';