$new = array();
foreach( $old as $key=>$value) {
	$key = strToLower($key);
	if(!array_key_exists($key,$new) {
		$new[$key] = $value;
	}
	else {
		throw new Exception('Duplicate Key Encountered');
	}

}

$lowerCaseKeys = array_map('strtolower', array_keys($array));
$duplicates = array_filter(array_count_values($lowerCaseKeys), create_function('$count', 'return $count > 1;'));
if (!empty($duplicates)) {
    throw new Exception('duplicate keys found: ' . implode(',', array_keys($duplicates)));
}
# Recreate the array with lower-case keys
$array = array_combine($lowerCaseKeys, array_values($array));

$lowerCaseKeyArray = array_change_key_case($array);
if (count($lowerCaseKeyArray) !== count($array)) {
    # You can extract the duplicate keys here as above, if you like
    throw new Exception('duplicate keys found!');
}

$new = array_change_key_case($old, CASE_LOWER);
if (count($new) < count($old)) {
    throw new Exception("Duplicate key encountered.");
}

foreach(array_keys($old) as $key) {
  $lower = strtolower($key);
  //if key is already lower case, do nothing
  if($key == $lower)
    continue;
  $value = $old[$key];
  unset($old[$key]);
  $old[$lower] = $value;
}