php生成随机数的三种方法

<?php
function randpw($len=8,$format='ALL'){
$is_abc=$is_numer=0;
$password=$tmp='';  
switch($format){
case 'ALL':
$chars='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
break;
case 'CHAR':
$chars='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
break;
case 'NUMBER':
$chars='0123456789';
break;
default :
$chars='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
break;
} // www.jb51.net
mt_srand((double)microtime()*1000000*getmypid());
while(strlen($password)<$len){
$tmp=substr($chars,(mt_rand()%strlen($chars)),1);
if(($is_numer <> 1 && is_numeric($tmp) && $tmp > 0 )|| $format=='CHAR'){
$is_numer=1;
}
if(($is_abc <> 1 && preg_match('/[a-zA-Z]/',$tmp)) || $format=='NUMBER'){
$is_abc=1;
}
$password.=$tmp;
}
if($is_numer <> 1 || $is_abc <> 1 || empty($password) ){
$password=randpw($len,$format);
}
return $password;
}
for($i=0 ; $i < 10; $i++){
echo randpw(8,'NUMBER');
echo "<br>";
}