What is the best way to generate random code that does not repeat in the database?

Question

I need to generate a random code of 5 characters, can not have repetitions. I managed to do as follows:

function testarcode($code){
    global $link;
    if ($code == ""){
        return false;
    }
    $comando = "SELECT * FROM tbl_usuario WHERE code='$code'";
    $query = mysqli_query($link, $comando);
    if(mysqli_num_rows($query) > 0){
        return false;
    }
    else{
        return true;
    }
}

function gerarcode(){
    $caracteres = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    $max = strlen($caracteres) - 1;
    $code = "";
    while (testarcode($code) == false){
        for($i=0; $i < 5; $i++) {
            $code .= $caracteres[mt_rand(0, $max)];
        }
    }
    return $code;
}

Although it is working I found my code very dirty. There is some way to make it easier and/ or simplified?

It is not a duplicate of other questions because in this question a code is asked on PHP and others, in MySql.

Answer 1

you can use the uniqid function that returns a prefixed unique identifier based on current time in millionths of a second.

echo uniqid();
// Saida: 5975b4239b793

or you can also pass a parameter to the function uniqid, for example:

echo uniqid(rand());
// Saida: 103550780059754516d29da

To get the first 5 characters use the sbstr function()

echo substr(uniqid(rand()), 0, 5);
// Saida: 43395