<?php
function caselessHasKey($arr, $str){
  foreach($arr as $k => $v){
    if(strcasecmp($str, $k) === 0) return TRUE;
  }
  return FALSE;
}

$a = Array("S" => 1, "I" => 2);
echo caselessHasKey($a, "s");
echo caselessHasKey($a, "d");
?>