PHP generates random strings (customized pure numbers, pure letters or a mixture of numbers and letters)

//Generate a random string
function rand_zifu($what,$number){
$string='';
for($i = 1; $i < ;= $number; $i++){
//Mixed
$panduan=1;
if($what == 3){
if(rand(1,2)==1){
$what=1;
 }else{
$what=2;
}
$panduan=2;
}
//Number
if($what==1){
$string.=rand(0 ,9);
}elseif($what==2){
//letter
$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);