actually it's nothing like 'hello world' versus "hello world".
The difference is this
$a = 'hello';
$b = 'world';
$c = $a . ' ' . $b . '!';
$d = "$a $b!";
$c is faster then $d because of this reason
the parser goes through $d three times:
1) reads it in
2) checks for variables
3) replaces variables with their values
the parser goes through $c once and accomplishes all of this on that single pass.
This means that what makes this grow by orders of magnitude is long strings not multiple executions of an assignment. So if you're pipping your entire html page into a variable and using interpolation you will slow the script down considerably by doing this.
You bring up some valid points allow me to address them:
Point) You would need hundreds of thousands of lines of code to slow this down apreciably.
Counter Point) That is true, if and only if, this is the only ineffiency in your code. However, people who tend to overlook the efficiency of their code will usually program with inefficient code a good percentage of the time. Therefor coding with an eye to effency is always desireable even if the individual steps look mute.
Point) You won't notice the difference.
Counter Point) That depends. We live in the world of the shared server. If we took a poll on phpbuilder I bet we would find that most people here are using a shared server, more then one site on a single computer, in one fashion or another. While each of these sites may have low traffic individually, when taken together their traffic can be astronomical. Therefor minimizing processor load can speed up the sites significantly. If you can time 7 microseconds off of each request and the server is handeling 1000 requests a minute you have just saved 7 seconds of processing time per minute. That is a noticable difference.
