Calling a modified value of the action destination based on form checkbox selection

I am generating a form using php. The form has several checkbox items that user can select. The php code looks similar to this.

function choice_gen ($msgs, $dest) { 
    $choices = explode("\n", $msgs); 
    $index = 1; 
    $return_string = "      <div class=\"checkbox_1\">\n";
    $return_string .= "         <form method=\"post\" action=\"" . $dest . "\">\n";
    foreach ($choices as &$choice) {
      $return_string .= "            <input type=\"Checkbox\" name=\"check_list[]\" id=\"C" . $index . "\" value=\"C" . $index . "\">\n"; 
      $return_string .= "            <label for=\"C". $index . "\">$choice</label>\n";
        $return_string .= "            <BR>\n\n";

       $index++;
    }
    $return_string .= "         <input type=\"Submit\" value=\"Next\" class=\"form-submit-button\">\n";
    $return_string .= "      </form>\n";
    $return_string .= "   </div>\n";

   if (!empty($_POST['check_list'])) {
      foreach($_POST['check_list'] as $check) {
         $select .=  $check; 
      }
   }
   return($return_string); 
}

$dest is a file name ('somefile.php'). I would like to change the action to "$dest?ch=$select" based on the user input. How do I do this using only PHP (no JS or Ajax etc.)?