How to check form input against PHP variable?

Your problem is this bit:

$input = $vCode

That is not comparing the variables. It’s assigning the value of $vCode to $input. If you want to compare the two values, you need to use == or ===.

$input == $vCode

It still wouldn’t work though, because you haven’t actually defined $input. You’re only defining it after the comparison, which isn’t much use.

So your logic needs to look like this:

$vCode   = 11111;
$message="";

if( ! isset( $_POST['submitbutton'] ) ) {
    $message="not submitted";
} else {
    $input = $_POST['vCode'];

    if ( $input == $vCode ) {
        $message="Success! You entered: " . $input;
    } elseif ( $input != $vCode ) {
        $message="OOps";
    }
}

Note that:

• We’re assigning $_POST['vCode'] to $input before checking it.
• We’re not bothering to check if $_POST['submitbutton'] is set, because the only reason that code would be running is if it was, because of the first condition.
• We’re using == to compare the value. We’re using == and not ===, because $vCode is an integer, while $_POST['vCode'] will be a string. If you’re actually getting $vCode from post meta, then it will also be a string, in which case you can use ===.

Regarding that last point, the best solution would be to convert them both to integers and use a strict comparison:

if ( (int) $input === (int) $vCode ) {