This:
$stmt = $dbh->prepare("SELECT thing FROM table WHERE color = :color");
$stmt->bindParam(':color', $someClass->getColor());
$stmt->execute();
yields this:
Runtime notice
Only variables should be passed by reference
though it still executes.
This:
$stmt = $dbh->prepare("SELECT thing FROM table WHERE color = :color");
$tempColor = $someClass->getColor();
$stmt->bindParam(':color',$tempColor);
$stmt->execute();
runs without complaint.
I don't understand the difference?
The description of PDOStatement::bindParam() states that it binds a PHP variable to a quesitonmark or named placeholder. Since you are trying to pass a class's method (even though that method does return a value) it is still not a variable name, hence the warning. You might want to look at PDOStatement::bindValue() to future-proof your code.
The second parameter of bindParam is a variable reference. Since a function return cannot be referenced, it fails to strictly meet the needs of the bindParam parameter (PHP will work with you though and will only issue a warning here).
To get a better idea, here's and example: this code will produce the same results as your second example:
$stmt = $dbh->prepare("SELECT thing FROM table WHERE color = :color");
$tempColor = NULL; // assigned here
$stmt->bindParam(':color',$tempColor);
$tempColor = $someClass->getColor(); // but reassigned here
$stmt->execute();
That won't be possible with a function return.
If you want to avoid assigning the value to a variable, you might be better off trying:
$stmt = $dbh->prepare("SELECT thing FROM table WHERE color = ?");
$stmt->execute(array($someClass->getColor()));
As others have mentioned, the error is caused because PDO::statement->bindParam expects param 2 to be a variable passed by reference.
If you really want to bind a value instead of a reference, you can use the PDOStatement::bindValue and then you code would look something like this:
$stmt = $dbh->prepare("SELECT thing FROM table WHERE color = :color");
$stmt->bindValue('color', $someObject->getColor());
$stmt->execute();
