Q:

Hash Password in PDO Error/Notice

If I run this I get the following error: 

Notice: Only variables should be passed by reference in /var/www/interface/register.php on line 11
      Success

I dont know how to fix that.  It's still successful and the data is hashed in the database, but I don't want this notice. 
$sql = " INSERT INTO users (username, password) VALUES (:username, :password)";
    $stmt = $conn->prepare($sql);

    $stmt->bindParam(':username', $_POST['username']);
    $stmt->bindParam(':password', password_hash($_POST['password'], PASSWORD_BCRYPT));

    if ($stmt->execute()) :
        die('Success');
    else:
        die('Fail');
    endif;

Thanks in advance. 

A:

You cannot do password_hash($_POST['password'], PASSWORD_BCRYPT) inside bindParam, because password_hash returns a string, do:
$password = password_hash($_POST['password'], PASSWORD_BCRYPT);
$stmt->bindParam(':password', $password);

If you wish to leave the values there use bindValue:
$stmt->bindValue(':username', $_POST['username']);
$stmt->bindValue(':password', password_hash($_POST['password'], PASSWORD_BCRYPT));

because it allows varibles by reference. 
Explanation:
bindParam expects a variable or const it can't be a primitive type such as a string or an int, ..., explicitly (ex: "some_hardcoded_string") neither can it be a function that returns one of this types.  
bindValue can receive references and primitive types as an argument.  

Examples for both:
$query->bindParam(':user', $user, PDO::PARAM_STR);

$query->bindValue(':pass', sha1($password), PDO::PARAM_STR);

SHA1 is returns a value, it could be a number 12345 (let's say for the sake of the example)
$query->bindValue(':pass', 12345, PDO::PARAM_STR); 

or a string. 
$query->bindValue(':pass', 'hashed_password', PDO::PARAM_STR);

retated questions:

Strict Standards: Only variables should be passed by reference in m_auth
PDO pass by reference notice? 
Strict Standards: Only variables should be passed by reference

