HTTP request not sending
You are lacking a space:

client.println((String)"GET /msg.php?action=send&amp;ID=2&amp;message=" += currentMessage += "HTTP/1.1");
                                                                                      ^


Your HTTP is running in to the content of your message ending up with a corrupt request. Also you are doing appending within your strings (+=) when you don't need to (+ is all that is needed). In fact, you don't even need that - just do multiple prints and reduce the hammering your heap is getting. Oh, and don't forget to use F():

client.print(F("GET /msg.php?action=send&amp;ID=2&amp;message="));
client.print(currentMessage);
client.println(F(" HTTP/1.1"));