Speed benchmark for PHP single-quote VS double-quote. Result: almose same.

php-quote-benchmark.php

<?php
$blah="blah"; $s=microtime(true); for($i=0;$i<100000;$i++) 'omgwtf'.$blah."\n"; echo microtime(true)-$s . "\n"; $s=microtime(true); for($i=0;$i<100000;$i++) "omgwtf{$blah}\n"; echo microtime(true)-$s . "\n";

https://groups.google.com/forum/#!topic/make-the-web-faster/3fvoLdgECMw

Nothing is wrong with the statement, you're just not interpreting it
very accurately...the difference is very small and slightly in the
favor of single quotes whenever a handful of variables are used.
However, when no variables are used, double quotes may be faster
because of implementation details in the engine.

These micro-optimizations can't get you more than 1 or 2 extra
requests per second done on a box...if that request has several tens
of thousands of variable interpolation operations. This is a LOT of
work for no real reward. Caching solutions and configuration tweaks
yield much higher results.