I read a LOT of crap here.
At first you are mistaken C with C++, which is a HUGE difference in performance as well as program design.
C is the fastest thing there is, you can easily use it to create webapps using the cgi library.
C++ is an OO language that's a tat slower, more like JAVA.
About C (not C++) versus PHP :
PHP is SLOW when you use it as an Apache module, average as a standalone app, and reasonable to good with Zend accelerator (cache).
It's very easy to learn, mature, but still, every time you start a thread (user request) you have the 8 megs of memory use for every thread.
With C, if you start an executable that's 5 kB, you use 5kB of memory. That's a lot of threads, huh ?
You say C is slow if used with Apache ?
That's because Apache is SLOW, use THTTP with FastCGI and be very surprised. You can have 500 concurrent users with CGI with no noticable system slowdown. Now try that with Apache (40 - 60 concurrent maximum).
Here's a good link to compare some languages (benchmark) :
http://scutigena.sourceforge.net/