It's only true if you write sloppy code. If you keep your variable names outside of the quotes (like you should), you will find no significant difference.
So, it's a myth
Anyways, even with sloppy code and the difference that you find, you can hardly talk about 'boosting' the performance. These differences are so minimal they're not really worth the trouble.
well, to contribute something to this topic. a mistake i often see is this:
for ($x=1; $x<count($array); $x++)
the count() will be performed with every loop, thus slowing down the script. So, it's better to store the size of the array in a variable first and use that in the for-loop