The performance difference has been irrelevant since at least January 2012, and likely earlier:
Single quotes: 0.061846971511841 seconds
Double quotes: 0.061599016189575 seconds
Earlier versions of PHP may have had a difference – I personally prefer single quotes to double quotes, so it was a convenient difference. The conclusion of the article makes an excellent point:
Never trust a statistic you didn’t forge yourself.
(Although the article quotes the phrase, the original quip was likely falsely attributed to Winston Churchill, invented by Joseph Goebbels’ propaganda ministry to portray Churchill as a liar:
Ich traue keiner Statistik, die ich nicht selbst gefälscht habe.
This loosely translates to, “I do not trust a statistic that I did not fake myself.”)