undefinedp - determine whether or not a given value 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]).

it has not yet been initialized.

it points to a destructed object.

it is a function (formal) parameter that corresponds to a missing actual argument.