$currentTime = date("Y-m-d H:i:s");

// We're looking for the last winner in the winners table
$res = $dbh->query("SELECT date FROM winnertable ORDER BY id DESC")->fetch(PDO::FETCH_ASSOC);

if(empty($res)){
    // If there is no last winner then the user wins
    $winner = true;
}else{
    // If there is already a last winner, we check the date of win, if superior to half an hour, the user wins
    if(strtotime($currentTime)-strtotime($res["date"])>=1800){
        // Last winner was half an hour ago : winner
        $winner = true;
    }
}

// if the winner flag is set to true, add the user id to the winners table along with the date of winning
if($winner){
    $dbh->query("INSERT INTO winnertable (id_main, date) VALUES ('".$_SESSION['id']."', '".$currentTime."')");
}