You don't have the mysql_query in your php, neither the variable sent. You have to get the post data, either by using Erel's example (file_get_contents) or by using the $_POST['yourquery'] type.
You simply connect to your database, but do nothing with the query sent from your b4a, which by the way, should use just the 'post' option, not the 'get' one. When the query is executed, a 'true' or 'false' will be returned as the result of the transaction, if successful or not. I still insist that you don't change Erel's php code. Just in the hc_responseSuccess (what so ever, I am not in front of a machine with b4a right now), you should parse the returning value (true or false) and thus know whether your 'insert' was successful. I have no better working example than the one provided at the mysql tutorial, which I use successfully for selecting, updating, inserting, deleting.