注:
As with all the variant arithmetic functions, the parameters for this function
can be either a PHP native type (integer, string, floating point, boolean or
NULL), or an instance of a COM, VARIANT or DOTNET class. PHP native types
will be converted to variants using the same rules as found in the constructor
for the VARIANT class. COM and DOTNET objects
will have the value of their default property taken and used as the variant value.
The variant arithmetic functions are wrappers around the similarly named
functions in the COM library; for more information on these functions, consult
the MSDN library. The PHP functions are named slightly differently; for example
variant_add() in PHP corresponds to VarAdd() in the MSDN documentation.
variant_sub