PHPのランダム文字列生成関数

functionrandom_string($length, $max=FALSE)
{
 if (is_int($max) && $max > $length)
 {
 $length = mt_rand($length, $max);
 }
 $output = '';

 for ($i=0; $i<$length; $i++)
 {
 $that = mt_rand(0,2);

 if ($that === 0)
 {
 $output .= mt_rand(0,9);
 }
 elseif ($that === 1)
 {
 $output .= chr(mt_rand(65,90));
 }
 else
 {
 $output .= chr(mt_rand(97,122));
 }
 }
 return $output;
}