This is an extremely open-ended question. Here are some possible answers:
- Large databases
MySQL will, if correctly configured (may also be dependent on OS configuration) support larger databases than you are likely to need.
PHP is unaffected by this, so as long as you don't try to fetch more than a reasonable number of rows at once, and your database is correctly indexed etc, PHP will have no impact on its performance
- Runtime performance of web applications
PHP is generally measured as being amongst the worst-performing languages for general purpose computing. Moreover, it also has a worse startup time than compiled things such as JSP, or ASPNET. The startup time problem can be mitigated by using one of the several open source, free or commercial PHP accelerators. These don't significantly affect runtime performance.
However, although PHP is not fast for general purpose programming, some of the features mean you can get generally good performance for web applications. This is because its string handling functions (etc) are generally pretty quick, even for largish strings.
Profiling your application is the only way to determine where the bottlenecks are.
Generally speaking, it is possible to create an inefficient application in any language or technology if you use it in an inappropriate way.
A lot of the time, raw performance doesn't matter too much, because either PHP is spending most of its time doing database queries (a common situation), or you're more concerned about scalability.
- Scalability
If used correctly, PHP can be extremely scalable. This is because it's a shared-nothing architecture, and doesn't rely on threaded platforms (whereas ASP does to a large extent, as does Coldfusion. Java and .NET do to a lesser extent). PHP's design to cope with prefork and CGI model servers, inadvertently caused it to be extremely scalable.
Note that your database still has to be scalable enough; for most applications this is going to be the critical point.
Note also that scalability is not the same as runtime performance, and it's extremely common to see a big performance drop if you move to a more scalable system. This is why you may see pages load more slower on that huge cluster than on your single-server development platform.
Mark
