undefinedp() - determine whether or not a given variable is undefined.

int undefinedp(mixed arg);

Return 1 if `arg' is undefined. 'arg' will be undefined in the following cases:

it is a variable set equal to the return value of a call_other to a non-existent method (e.g. arg = call_other(obj, "???")).
it is a variable set equal to the return value of an access of an element in a mapping that doesn't exist (e.g. arg = map[not_there]).