Sample sweepstakes for Gravity Forms

sweepstakes.php

<?php

//Change gform_pre_submission_1 to gform_pre_submission_(your form # here)
add_action( 'gform_pre_submission_1', 'sweepstakes_winner_check' );
function sweepstakes_winner_check( $form ) {

    //Array of codes for the first listed prize
    $prize_one = array('aaaa','bbbb','cccc');

    //Array of codes for second listed prize
    $prize_two = array('rrrr','tttt','zzzzz');

    //Add more as desired

    //Change 2 to the field number the code is put into
    $submitted_code = $_POST['input_2']; 

    $winner_or_not = 'no'; //Start off with "not a winner" by default
    $prize = 'None'; //start off with "no prize" by default

    if(in_array($submitted_code,$prize_one)) {
        $winner_or_not 'winner';
        $prize = 'A brand new car!';
    } 
    elseif(in_array($submitted_code,$prize_two)) {
        $winner_or_not = 'winner';
        $prize = 'Lifetime supply of toothpaste!';
    }

    //Input 3 is a hidden field that stores if the person won or not
    $_POST['input_3'] = $winner_or_not;
    
    //Input 4 is a hidden field that stores the prize won
    $_POST['input_4'] = $prize;

}