php生成随机生存字符串

/****************************************
*  @Explain:  随机生存字符串
*  @Remarks:
*  @Param:       $len:生存字符串的长度， $ul:大小写的区别，1：大小写混合，2：小写，3：大写， $letter:字符性质，1：数字和字符混合，2：纯字符，3：纯数字
*  @Return:
****************************************/
function randStr($len=0, $ul=1, $letter=1){
   $len            = (int)$len;
   if($len < 1){return '';}
   $strArr             = array('1','2','3','4','5','6','7','8','9','0','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
   if($letter == 3){
      $strArr          = array_slice($strArr, 0, 10);
   }elseif ($letter == 2){
      $strArr          = array_slice($strArr, 10);
   }
   $max            = count($strArr) - 1;
   $result             = '';
   for ($i=0;$i<$len;$i++){
      $key         = rand(0, $max);
      $result          .= $strArr[$key];
   }
   if($ul == 2){
      $result    = strtolower($result);
   }elseif ($ul == 3){
      $result    = strtoupper($result);
   }
   return $result;
}