If there is a syntax error in my switch control structure, my script returns a message pop-up 'The server returned an invalid or unrecognized response.', 
It seems that the syntax of the switch control structure is not checked properly before processing the script. 
this is my invalid switch structure (missing a 'case' statement):
$varX = "a";
switch($varX){
  "a":
    echo("a");
    break;
  case "b":
    echo("b");
    break;
}

check it out!