error checking a null array

hi, I have checkboxes beging dynamically generated. to seperate tasks in error checking I have added he arrays not just to $_POST but to $_POST[cb] so names (derived from question numbers) are for example: $_POST[cb][7] A dump of $_POST would therefore include something like ["cb"]=> array(1) { [7]=> array(1) { [0]=> string(3) "124" } } So I did this: //error checking for checkboxes foreach ($cb as $cbkey => $cbvalue) {

      foreach($cbvalue as $cbkey2=>$cb_answers)
      {
         if( !array_key_exists($_POST[$cbkey], $cb));
         {
         $message[$cbkey]=1;
         }
       }

} This almost works. The problem is that if someone doesn't check a checkbox, NOTHING related to the checkbox gets sent to $_POST; if I insert a hidden value of something, then that hidden value gets passed whether the user inputs something or not, since I need an entirely server-side solution. Is there a way to send something to tell the error checker whether for example $_POST[cb][7] is empty or null, so I can set the error message to one if it's empty? Dump of $_POST with no checkboxes checked $_POST: array(4) { ["action"]=> string(7) "process" ["cat"]=> string(1) "2" ["rs"]=> array(3) { [5]=> string(0) "" [6]=> string(0) "" [9]=> string(0) "" } ["b"]=> string(8) "Continue" } Dump of $_POST with checkboxes checked array(5) { ["action"]=> string(7) "process" ["cat"]=> string(1) "2" ["cb"]=> array(2) { [7]=> array(5) { [0]=> string(1) "3" [1]=> string(3) "124" [2]=> string(3) "125" [3]=> string(3) "127" [4]=> string(3) "131" } [8]=> array(3) { [0]=> string(3) "141" [1]=> string(3) "145" [2]=> string(1) "4" } } ["rs"]=> array(3) { [5]=> string(0) "" [6]=> string(0) "" [9]=> string(0) "" } ["b"]=> string(8) "Continue" } Thanks so much In advance