<?php

$int = (isset($_GET['int']) && is_numeric($_GET['int']) && $_GET['int'] >= 3 && $_GET['int'] <= 9) ? $_GET['int'] : rand(3, 9);
$strings = [];
$abc = ['a','b','c','d','e','f','g','h','j','k','m','n','p','q','r','s','t','u','v','w','x','y','z'];

for($i = 1; $i <= $int; $i++) {
    $strings[] = get_letter(rand(1, 10000), rand(0, 1) == 0 ? 0 : 1);
}

function get_letter($i, $modus) {
    global $abc;

    if($i > count($abc)-1) {
        return get_letter($i-count($abc), $modus);
    }

    return ($modus == 0) ? $abc[$i] : strtoupper($abc[$i]);
}

$result = '';

foreach($strings as $string) {
    $result .= $string;
}

echo $result;