Only variables should be passed by reference in… on line 13 Fail

Question

I\'m practicing building a PHP registration form script for a website. I have done this code, but when I click the submit button I get the notice: Only variables should be p

Answer 1

$result->bindParam(':id', $id, PDO::PARAM_INT);

if string, you need write:
$stmt->bindParam(':username', $_POST['username'], PDO::PARAM_STR);