Which is better on performance: double quoted strings with variables or single quoted strings with concatenations?

Question

I know that using single quotes around a string in PHP is faster than using the double quotes because PHP doesn't need to check for variable presence in the single quoted string. My question is which will perform better:

A) A double quoted string with variables present:

echo "foo bar $baz";

or

B) Single quoted with a concatenated variable:

echo 'foo bar ' . $baz;

Answer 1

I did a benchmark of this on a blog I was working on a while ago. However, as I've come to realize there are a lot of variables. Chief among them are:

1. How many concatenations are you doing? Each time you do a concatenation PHP re-parses the entire string (or so I've been told). So 1 concatenation may be faster, but 6 may be considerably slower.
2. Data type. Though I haven't tested this one myself personally I've been told that the data type being concatenated matters as well, though I'm not sure how much.

Over all I'd say it really isn't that big of a deal for you to actually worry about it. Generally speaking it's only going to make a noticeable difference if you're writing a huge site (think MySpace, Facebook, Flickr, etc) and usually by that point you have so much hardware behind you that the single vs double quotes thing becomes irrelevant again.

Personally I'd say there's far more important things that will impact performance in a much more substantial way (caching, sql optimization, auto loading to prevent unnecessary includes, etc).

I personally choose single quotes nearly every time, but not for speed. I do because I think it's more readable. And that to me is important.

Answer 2

In all honesty, I don't think you need to be scrutinizing this particular convention when looking to save performance. But, for what it's worth, I think I recall reading that variables within strings are less-efficient (mind you, it's negligible and in my opinion not an issue) than concatenation.