This is my first really technical post on my blog here, so bear with me if you're not from the techie crowd (in fact, you may just want to skip this one, your brain cells don't need the stress). Single and double-quoted strings are an oft-argued entry PHP topic; however they are used so often that getting to the bottom of this issue could have significant performance improvements. Our goal in designing high performance websites is to optimize the common conditions and avoid the edge case gotchas. With that in mind, let's take a look at this issue.
When I started PHP 4, (which meant moving from the illustrious PERL) my good friend and ZCE Justin Beasley (@justbeez) was all about the single-quoted strings. I pretty much took his word and the documentation seemed to support them as a better go-to, but is it still that way in modern PHP? TL;DR is yes, but not by much. PHP has gotten a lot smarter in recent versions, but there still is overhead to variable embedding.
All of my loads of PHP Bench actually showed double-quoted strings performing slightly better than single (surprising!). Unfortunately he also does not note which version of PHP he is using. But, on the other side of the aisle, Micro Optimization notes that double quoted-strings are slower. Classy Llama (who also does not show which PHP version he's working on) shows a significant performance detriment to actually using an embedded variable over concatenation, and really if you aren't going to embed variables (other than avoiding escaping strings with lots of apostrophes) why would you want to use a double quoted string? As Classy Llama points out, there is a readability benefit as well to using concatenation and Zend.
Now, I'm thinking this is a version issue (yet another one who doesn't list his version), but this guy noticed that if you ever use a dollar sign that is unattached to a variable in a double-quoted string, there is a huge performance penalty. Even though this is an edge case, it's a great reason to use concatenated single-quoted strings. However Jeff Moore is suggesting that all the tests thus far are missing the boat because the actual embedding process will be done on interpretation which is not being considered in these processes. So, perhaps this is all meaningless. When it comes down to it, I think concatenated values are easier to read, and it's quite possible single-quoted strings perform better.