On that typo... it shouldn't matter since the error follow 'return;' and anything that follows 'return;' from a function is ignored.
The bigger problem that I see is the complete uselessness of the function itself. Simply adding two numbers does NOT require a function call:
$thisnum = 7;
$thatnum = 12;
print($thisnum + $thatnum);
Screw SAMS and their sorry-ass books which, IMHO, deliver nothing but page after page of trivial, useless, non-real-world coding examples. I've spent considerably more than 24 hours on their "Teach Yerself C in 24 Hours", and I'll be damned if I'm a better C programmer because of it.
Just my too sense...
Have a nice day!