php生成随机字符串可指定纯数字、纯字母或者混合的

function rand_zifu($what,$number){
$string='';
for($i = 1; $i <= $number; $i++){
//混合
$panduan=1;
if($what == 3){
if(rand(1,2)==1){
$what=1;
}else{
$what=2;
}
$panduan=2;
}

//数字
if($what==1){
$string.=rand(0,9);
}elseif($what==2){
//字母
$rand=rand(0,24);
$b='a';
for($a =0;$a <=$rand;$a++){
$b++;
}
$string.=$b;
}
if($panduan==2)$what=3;
}
return $string;
}
echo rand_zifu(3,10);