how to compile c program using php script in ubuntu

how to compile c program using php script in ubuntu

I am developing an online testing system through which we can conduct
online programming contests. For the same i want to invoke the C compiler
from the php script using the functions like system(), shell_exec(),
exec() etc. But the function is not creating a.out file when i wrote g++
a.cpp -o a using terminal it worked.
But this is not working system("g++ a.cpp -o a",$as); //a.cpp is inside
www folder