I'm talking about the performance increase here. From all I know you can echo variables in double quotes ("), like so:
<?php
echo "You are $yourAge years old";
?>
But single quotes will just return You are $yourAge years old. But what about performance differences? I've always gone by the rule that single quotes are faster because the PHP interpreter doesn't have to search through the string for variables. But I'm seeing more and more blog and forum posts on the web saying differently.
Does anyone actually have any information on this subject? Perhaps benchmark tests or something?
According to the PHP Benchmark, the difference is extremely negligible:
single (') quotes. 20 bytes Text and 3x a $ : $tmp[] = 'aa $ aaaa $ aaaa $ a'
235 µs
double (") quotes. 20 bytes Text and 3x a $ : $tmp[] = "aa $ aaaa $ aaaa $ a";
226 µs
Even if the differences were a multiple of what they are, they would not be relevant for real-life performance IMO. Database and file operations will take dozens, if not hundreds of times more time. That's not to say your question isn't totally valid, but it's not a big deal when optimizing your code.
Readability is much, much more important.
I haven't benchmarked it myself. I've read that single quotes are faster from places like http://phpbench.com. I actually read today at PHP Best Practices that double quotes is actually faster thought they don't provide any sources :-/.
Benchmarks provided here are pretty interesting, informative, and answer your question.
The PHP Benchmark site doesn't show much difference (second test from the bottom) between them at all. I mean, it's very slightly faster, but it's hardly something that you need to worry about, I'm sure.
If you're concerned about the performance of your site, there's going to be much better places to look than taking microseconds off your print calls.