They are all correct.
When I was an intern, I emailed a C compiler developer, to ask a question that had occurred to me regarding optimization: which is faster, ++i or i++? Assuming either form will work in my case, as in the increment expression in a for() loop. His response (paraphrased):
"By emailing me this question, you have already wasted more computing resources than you will ever save by choosing one form over the other during your entire programming career."(I'm still not sure if he meant to emphasize that the performance difference between the two expressions was extremely small, or that he didn't think very highly of my career prospects. I'll prefer to assume the former.)
A list of performance factoids like those listed by Alex are missing the guidance and wisdom that software developers need to judge their importance. All of the responses from other bloggers are similarly qualitative, instead of quantitative.
I know that it's hard to make quantitative statements with regards to optimization.
- How much benefit can I get by replacing print with echo? It depends on how much printing you do in a given application -- and also what else you're doing in that application.
- Can I benefit from caching page output or results of resource-consuming functions? Probably, but not if the content is 100% dynamic and must be re-calculated for each request.
- Which of these micro-optimizations should I employ with greater priority? Which is the best use of my development time?
Finding the best way to optimize your code is your job as a software developer. You must use scientific measurement, as well as good judgment, experience, and intuition to get your job done most effectively.