How does PHP realize that the letters in the verification code are randomly included from a-z in upper and lower cases.

The code is as follows:

function createRandomStr($length){

$str = array_merge(range('a ','z'),range('A','Z'));

shuffle($str);

$str = implode('',array_slice($ str,0,$length));

return $str;

}

echo createRandomStr(4); >