MySQL - inserting 70000 random unique strings efficiently

I'm working on a project, in which I should generate at least 70000 codes which contain 8 alphanumeric characters. The codes must be unique. currently I am using php to generate these codes with the following function :

function random_unique_serial($length, PDO $conn) {
    $codeCheck=FALSE;
    while (!$codeCheck) {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
        $charactersLength = strlen($characters);
        $randomCode = '';
        for ($i = 0; $i < $length; $i++) {
            $randomCode .= $characters[rand(0, $charactersLength - 1)];
        }
        $sql = "SELECT * FROM codes WHERE code=:code";
        $st = $conn->prepare($sql);
        $st->bindvalue(":code", $randomCode, PDO::PARAM_STR);
        $st->execute();
        $count = $st->rowcount();
        if ($count==0) {
            $codeCheck=TRUE;
        } else {
            $codeCheck=FALSE;
        }
    }
    return $randomCode;
}

As you see this codes checks the database for every single code generated to make sure it is not a duplicate. This should work theoretically. However this is very slow and causes the request to time out. I tried increasing execution time but that also didn't help.

Then I decided to use a database side approach and used this solution : Generating a random & unique 8 character string using MySQL

This is also very slow and some of the generated codes are less than 8 characters long.

could you please suggest a better solution?