How to loop to check either the ID is exist or not in mysql database with PHP after a random function's id is generated?

I have a database called Customers with Unique ID, so each time insert from php to database I need to generate a new ID for the new customer from a random function but I need to check the id that generated is exists in database or not.

This is the php random function:

function randomDigits($length){
    $digits = "";
    $numbers = range(0,9);
    shuffle($numbers);
    for($i = 0;$i < $length;$i++)
       $digits .= $numbers[$i];
    return $digits;
}

I tried to pass random length like this:

    $cusid = randomDigits(10);
    $generate_customer_id = "";

    $query_customer_id = dbQuery("SELECT customer_id FROM customers WHERE customer_id = '$cusid'");
    $count_customer_id = $query_customer_id->rowCount();

    if($count_customer_id == 1)
    {

        $generate_customer_id = randomDigits(10);

    }
    else
    {
        $generate_customer_id = $cusid;
    }
    $query = dbQuery("INSERT INTO customers(`customer_id`,`username`,`password`,`email`,`first_name`,`last_name`,`dob`,`id_card`,`family_book_id`,`address`,`district`,`province`,`phone`,`mobile`,`father_name`,`mother_name`,`bank_account_no`,`bank_account_name`) VALUES('$generate_customer_id','$username','$password','$email','$firstname','$lastname','$dob','$idcard','$familybook','$address','$district','$province','$homephone','$mobilephone','$fathername','$mothername','$bankno','$bankname')");

    if($query == true)
    {
        $insert_referrer = dbQuery("INSERT INTO referrers (`parent_id`, `customer_id`, `category_id`, `created`,`modified`) VALUES(NULL, '$generate_customer_id', 2, NOW(), NOW())");
        if($insert_referrer == true)
        {
            echo "OK";
        }
        else
        {
            echo "There is an error please check your information and try again referrers!";
        }
    }
    else
    {
        echo "There is an error please check your information and try again customers!";
    }

But data return from my AJAX function, if return error I need to click the save customer button many times then i will return success and then data successful saved to database.

But I want user to click only one time then data will insert.

How to implement this please help!