Crossplatform desktop PHP ?

Hello & Thanks ,
Is there such a thing as "Crossplatform desktop PHP" ?
Normally I program in HotBasic (freeware programs) .
But I am interested in crossplatform .
So it seems like javascript/html/php are quite platform independent .
Today I noticed a Php-Gtk crossplatform opensource package .
I would like to write programs that run in Browsers (IE , chrome , firefox , etc.) .
So I am looking for the smallest Desktop footprint possible ,
to develop php/html/javascript programs .
And more importantly ,
I am looking for the smallest Desktop footprint possible ,
to run these programs in .

Any advice/direction you can give me will be greatly appreciated .

Thanks...Vernon