无法将_ $ GET [＆＃39;用户名＆＃39;]发送到数据库中？

Question

由于某些奇怪的原因，我无法在数据库中插入用户名，但是当我将值更改为任何整数时，只要我点击它就可以工作。

$postid = $_POST['postid'];
$userid = $_GET['username'];
$result = mysqli_query($dbh,"SELECT * FROM user_images WHERE id=$postid");
$row = mysqli_fetch_array($result);
$n = $row['likes'];
mysqli_query($dbh,"INSERT INTO likes(username, postid) VALUES($userid, $postid)");
mysqli_query($dbh,"UPDATE user_images SET likes=$n+1 WHERE id=$postid");
echo $n+1;
exit();