Q:

Cannot pass parameter 2 by reference error in php PDO

I am getting the error:
Cannot pass parameter 2 by reference in.....
in this line...
$stmt1->bindParam(':value', $_SESSION['quantity'.$i] * $_SESSION['price'.$i], PDO::PARAM_STR);   

What is wrong with code above ??

A:

I'd say it's the typical confusion between PDO:bindParam() and what you probably intended to use: PDO:bindValue().

PDOStatement::bindParam
Binds a PHP variable to a corresponding named or question mark
  placeholder in the SQL statement that was used to prepare the
  statement. Unlike PDOStatement::bindValue(), the variable is bound
  as a reference and will only be evaluated at the time that
  PDOStatement::execute() is called.

PDOStatement::bindValue
Binds a value to a corresponding named or question mark placeholder in
  the SQL statement that was used to prepare the statement.

A:

It expects the second paramter to be a variable which can be passed by reference.  Assuming $stmt1is a PDO statement then, as the docs for bindparam say

Unlike PDOStatement::bindValue(), the variable is bound as a reference
  and will only be evaluated at the time that PDOStatement::execute() is
  called.

Your second param is an expression ($_SESSION['quantity'.$i] * $_SESSION['price'.$i]) not a variable.  Since you appear to want to evaluate the exptression now, I guess you should used bindValue() instead.

