Validate Promo Code for one time use PHP/My SQLi

I’m very new to the whole MySQLi thing. I have a basic understanding of PHP. Anyway, I’ve searched around here and just haven’t been able to get a solid answer.

Basically, I have a form where a person can enter name, email and promo code. The form validates the name and email but when it comes to the promo code, that’s where I’m getting stuck.

I have a database that has two columns. One is for the codes and the other is for a “used” column – eventually I need to be able to write a “1” to that column when a unique code has been used so it cannot be used again. I’m trying to use some code I found on here, FYI.

Here is the PHP (after connecting) to the database:

if(isset($_POST['sponsorcode']) && !empty($_POST["sponsorcode"])){
   $sponsorcode = mysqli_real_escape_string($link,$_POST['sponsorcode']);
   $query = "SELECT 'sponsorcode' FROM 'teachercodes' WHERE sponsorcode = '$sponsorcode'";
   $result = mysqli_query($link, $query) or die(mysqli_error($link));
   $option = "";
   if(mysqli_num_rows($result)>0){
       while($row=mysqli_fetch_array($result)) {
       $option = "<option value='{$row['codes']}'>{$row['codes']}</option>";
}

Any tips would be GREATLY appreciated! Thanks.

No reason to perform your task as two separate steps. Simply mark the sponsor code as used in the teachercodes table. If the update affected any rows (i.e. mysqli_affected_rows returns 1 or more) then it hasn't been used before and is a valid sponsor code. Something like this:

// Make sure a sponsor code was provided
if (isset($_POST['sponsorcode']) && !empty($_POST['sponsorcode'])) {

    // Escape the sponsor code to prevent SQL injection
    $code = mysqli_real_escape_string($link, $_POST['sponsorcode']);

    // Mark sponsor code as used if possible 
    $sql = 'UPDATE teachercodes SET used=1 WHERE sponsorcode="' . $code . '"';
    mysqli_query($link, $sql) or die(mysqli_error($link));

    if (mysqli_affected_rows($link)) {
        // Sponsor code hasn't been used before and is valid
    }
}