Detect if browser request is a prefetch or prerender using PHP

gistfile1.php

if ( (isset($_SERVER["HTTP_X_PURPOSE"]) and (strtolower($_SERVER["HTTP_X_PURPOSE"]) == "preview")) or 
(isset($_SERVER["HTTP_X_MOZ"]) and (strtolower($_SERVER["HTTP_X_MOZ"]) == "prefetch")) ) {
    // Request is a prerender or prefetch
} else {
    // Request is 'normal'
}