function getHeaders()
{
    $headers = array();
    foreach ($_SERVER as $k => $v)
    {
        if (substr($k, 0, 5) == "HTTP_")
        {
            $k = str_replace('_', ' ', substr($k, 5));
            $k = str_replace(' ', '-', ucwords(strtolower($k)));
            $headers[$k] = $v;
        }
    }
    return $headers;
}  

The PHP function getallheaders() only works on Apache, and only if Apache has been installed as a module.

It turns out that this information is within the $_SERVER global variable, and can be parsed out.

This code is useful if, for instance, you want to differentiate between user browser requests and requests made through AJAX. Some of the JavaScript frameworks (jQuery, Prototype, YUI, MooTools) send a special header,

X-Requested-With: XMLHttpRequest

whenever a request is made. I have put this to use in my phpFreeChat chat box to allow channels to be specified in the URL as GET parameters.

via request headers php – bluehostforum.com.