$query = "SELECT * FROM `table_name` WHERE `submission_id` = '$submission_id'";
$sqlsearch = mysql_query($query);
$resultcount = mysql_numrows($sqlsearch);

if ($resultcount > 0) {

    mysql_query("UPDATE `table_name` SET
                                `name` = '$name',
                                `email` = '$email',
                                `phone` = '$phonenumber',
                                `subject` = '$subject',
                                `message` = '$message'
                             WHERE `submission_id` = '$submission_id'")
     or die(mysql_error());

} else {

    mysql_query("INSERT INTO `table_name` (submission_id, formID, IP,
                                                                          name, email, phone, subject, message)
                               VALUES ('$submission_id', '$formID', '$ip',
                                                 '$name', '$email', '$phonenumber', '$subject', '$message') ")
    or die(mysql_error());

}
?>

You're done.  Name the script as your thank you php file and upload it to match the exact detail that you have set in your Custom Thank You URL

Here's the complete script:


// This function will run within each post array including multi-dimensional arrays
function ExtendedAddslash(&$params)
{
        foreach ($params as &$var) {
            // check if $var is an array. If yes, it will start another ExtendedAddslash() function to loop to each key inside.
            is_array($var) ? ExtendedAddslash($var) : $var=addslashes($var);
            unset($var);
        }
}

// Initialize ExtendedAddslash() function for every $_POST variable
ExtendedAddslash($_POST);

$submission_id = $_POST['submission_id'];
$formID = $_POST['formID'];
$ip = $_POST['ip'];
$name = $_POST['name'];
$email = $_POST['email'];
$phonenumber = $_POST['phonenumber13'][0] ."-". $_POST['phonenumber13'][1];
$subject = $_POST['subject7'];
$message = $_POST['message6'];

$db_host = 'db hostname here';
$db_username = 'db username here';
$db_password = 'db password here';
$db_name = 'name of your database';

mysql_connect( $db_host, $db_username, $db_password) or die(mysql_error());
mysql_select_db($db_name);

// search submission ID

$query = "SELECT * FROM `table_name` WHERE `submission_id` = '$submission_id'";
$sqlsearch = mysql_query($query);
$resultcount = mysql_numrows($sqlsearch);

if ($resultcount > 0) {

    mysql_query("UPDATE `table_name` SET
                                `name` = '$name',
                                `email` = '$email',
                                `phone` = '$phonenumber',
                                `subject` = '$subject',
                                `message` = '$message'
                             WHERE `submission_id` = '$submission_id'")
     or die(mysql_error());

} else {

    mysql_query("INSERT INTO `table_name` (submission_id, formID, IP,
                                                                          name, email, phone, subject, message)
                               VALUES ('$submission_id', '$formID', '$ip',
                                                 '$name', '$email', '$phonenumber', '$subject', '$message') ")
    or die(mysql_error());

}
?>