php简单生成随机数的方法

<?php
/**
 *生成随机数，可用户验证码
 *@param
 */
function randStr($m = 5) {
  $new_str = '';
  $str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwsyz0123456789';
  $max=strlen($str)-1;
  for ($i = 1; $i <= $m; ++$i) {
    $new_str .=$str[mt_rand(0, $max)];
  }
  return $new_str;
}
//测试
$str = randStr(6);
echo $str;