I love both philippa and Matilda. Actually we were all set to call our dd Matilda, then realised that here in Australia it is like number 18 in popularity, and that theres connotations with 'Waltzing Matilda' and various characters callled Matilda - so that ruined the name for us. I f we were still living in the UK we'd DEFINITELY have a Matilda.
Philippa is lovely and timeless and Pip//Pippa/Pippy/Phil are lovely nn's for it. People would know how to say and spell the name, but it's not popular enough that it's likely there'd be lots of Philippas in her class.