Sara Golemon, inspired by an IRC discussion has gathered together some of her thoughts on "using PHP's string interpolation without using an optimizer".
She explains how a simple string (an echo statement) is interpreted into a simple compilation structure. Her next step, though (placing a variable inside a string) yields something that seems more complex than it should be. A concatination example simplifies things down a bit, but, oddly enough, it gets even better when a comma is used instead of a period to concatinate. She also gives an example of a heredoc statement that doesn't conform to the interpolation standards you'd think.
Why does this happen? Because there are about a dozen ways that a variable can be hidden inside an interpolated string. Similarly, when looking for a heredoc end-token, the token can be an arbitrary length, containing any of the label characters, and may or may not sit on a line by itself. Put simply, it's too difficult to encompass in one regular expression.
She specifically mentions the APC caching system and its built-in optimizer to help with some of these issues. It pulls the interpolations back down to a size they should be and anticipating operations by pre-resolving things like constants and scalar expressions.
Of course, not everyone can install this pacakge, so she suggests an alternative:
You can still avoid 90% of the INIT_STRING/ADD_STRING dilema by simply using single quotes and concatenation (or commas when dealing with echo statements). It's a simple trick and one which shouldn't harm maintainability too much, but on a large, complicated script, you just might see an extra request or two per second.