If the incoming PHP possible requests can be the following
1)http://wwww.test.com/api.php?device_id=xxxx&user_name=xxxx&pass_word=xxxx&num_request=xxxx&num_site=xxxx;
how do I extract the user_name field from the above in my api.php code?
Those values are passed via a standard GET request accessible like this:
$deviceid = $_GET['device_id'];
Be aware that this value is unsanitized and open to modification from the user which could ultimately end up with SQL injection depending on your usage of the value.
You can use the $_GET superglobal to acess querystring values
$user_name = $_GET['user_name'];