Alias variable vs multiple use of getter

I'd prefer the second approach, but with a better variable name.

• Don't repeat the logic for obtaining the manager (DRY). If this changes, you'll only need to change one place instead of three.
• Decouples the logic of operating on the manager from obtaining the manager. Extracting it to its own method is only a small step.
• Assuming you use a good name (not $em) it's more readable.
• It's potentially faster, but that's only relevant if this is executed in a tight loop, and fetching the manager is a bottleneck. I expect a flush or persist operation to be much more expensive than fetching the manager. So I doubt performance is measurably better.

Introducing local variables is often a good idea, but generally not due to performance improvements.

Chained methods are awesome, but I almost always go for the variable approach. Other than what CodesInChaos already mentioned, it's just a lot quicker to interject some code and do some debugging / error handling.

For example, in your code it would probably make sense to check if you've actually instantiated the Entity Manager before you start using it:

$em = $this->getDoctrine()->getManager();

if(!$em) {
    throw new Exception("Hey, where's my entity manager?");
}

$em->persist($currency);
$em->persist($user);
$em->flush();