[PHP] 随机生成 6 位字符串问题

function createStr($num = 700, $len = 6){
    $numbers = range(0, 9);
    $letters = range('a', 'z');
    $uppers = range('A', 'Z');

    $elements = array_merge($numbers, $letters, $uppers);

    $n = 0;
    $strings = [];

    for($i = 0;$i < $num;$i++){
        $str = '';
        $keys = array_rand($elements, $len);
        foreach($keys as $key){
            $str .= $elements[$key];
        }

        if(in_array($str, $strings)){
            $i--;
            unset($str);
        }else{
            $strings[] = $str;
            unset($str);
//            echo $i. '->' .$str. "\r\n";
        }

        $n++;
    }

    print_r($strings);exit;
//    echo "\r\n\r\n".$n;
}