Abstration in programming.. ie modelling the solution to the problem vs. modeling against the machine... Too many programmers make the mistake of modelling the machine code rather than solving the problem...
How many of us have learned not to write our own classes but to extend (meaning inheritance) a class. (and hence why interfacing in PHP5 and the PEAR packages is such a boon) .
PHP is still interpreted, not compiled though and your instructor might cite that against you since PHP is yet to be full-fledged OOP (and never will be according to Zeev.... )