Why Does `bindParam` Fail with Constant Values in PDO?

bindParam Error with Constant Values in PDO

When attempting to use bindParam with a constant value, such as PDO::PARAM_NULL, developers may encounter the error "Cannot pass parameter 2 by reference."

Code Snippet with the Error

try {
    $dbh = new PDO('mysql:dbname=' . DB . ';host=' . HOST, USER, PASS);
    $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $dbh->setAttribute(PDO::MYSQL_ATTR_INIT_COMMAND, "SET NAMES 'utf8'");
}
catch(PDOException $e)
{
    ...
}
$stmt = $dbh->prepare('INSERT INTO table(v1, v2, ...) VALUES(:v1, :v2, ...)');
$stmt->bindParam(':v1', PDO::PARAM_NULL); // --> Here's the problem

Solution

The error occurs because bindParam takes a variable by reference and doesn't retrieve a value when called. The correct method for binding a constant value is bindValue.

Corrected Code

$stmt->bindValue(':v1', null, PDO::PARAM_INT);

Note:

Avoid using bindValue(':param', null, PDO::PARAM_NULL), as it may not be reliable.

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