C has a function called assert(booean test), such
that if the test if false,  execution is halted
and a callstack is printed, including file and line
numbers. If the test is true, no action is taken.
It is useful for locating bugs in code.

For example, a function that takes argument int x
which should be positive can have in it's code:
   assert( x > 0 );

I'd very much like to see such a feature added to
PHP4, as PHP3 is sorely lacking in this area.