It may be obvious to everyone else but...
If you want to write your own COM DLL in MSVC++6 and you want to pass it a string, you need the following in your .idl file:
HRESULT function_name([in] BSTR parameter_name,
[retval, out] BSTR * retval);
retval is the result of your function as a string to be passed back.
BSTR is an unsigned short *, so if you want to use your string with STL string etc. you may need to convert parameters to and from char *.
The burden of my message is that from PHP
$comThing = new COM("comThing.comThing");
print $comThing->function_name("Jeremy");
Jeremy will be marshalled as wide chars, which match BSTR.