zebaron is correct, we use single quotes for literals as often as possible, and double-quotes when needed (to evaluate internal expressions) or even when not needed just because. This is a recommendation, not a strict rule.
We are (or I am, at least) very aware of the behavior of these two quoting types, along with the so-called performance difference.
The logic in the loose rules is that the [i]performance difference between double- and single-quotes is negligible[/i], so worrying too much about it is simply nitpicking and we shouldn't waste time on it. See: http://spindrop.us/2007/03/03/php-double-versus-single-quotes/
More specifically, the fastest possible thing is single-quotes with concatenation, double-quotes with concatenation is [i]nearly exactly the same[/i], and the slowest is double-quotes with inline variables.
So really, the rule that makes the most sense is screw worrying what kind of quotes you use, but only use inline-variables in double quotes when necessary for readability when building long value-ridden strings.
Still, even remembering that it doesn't matter *much*, try to use single quotes for string literals anyway, because, well, why not? And we might shave 0.0001 seconds off each page load. Whoop-de-doo!