in reply to A better non-existant string...
Single- and double-quoted strings both have to be parsed at compile time. Both get compiled into constants and so there is no speed difference between the two at run time.
Now a string like "We have $n things" actually gets compiled into "We have ".$n." things". The string is not reparsed each time you hit that line of code.
I had to resort to a 3200-byte string and finally was able to detect at 20% slow-down for double-quoted strings. This means that if your script has four thousand 3200-byte strings enclosed in double quotes, then you could make it start up 0.2 seconds faster by changing all of those double quotes to single quotes. No matter how long the script ran, no further speed benefit would be achieved.
So "optimizing" 0-byte strings to use single quotes instead of double quotes is just silly. The added complexity to your mental decision tree probably costs more time than you ever save. (:- tye (but my friends call me "Tye")