byron19,
Firstly, welcome here to the forum.
I'm sure you already know that this is a complicated scenario you both face.
Right at the outset my honest advice is clear, and has to be that you both must seek professional advice from a specialist immigration advisor who has experience in this area, understands the relevant caselaw and will fully understand the specific background details and circumstances to help you both in knowing the benefits and the risks of available options.
In my view it's imperitive that you are both totally clear from the start what you're are getting into.
Many others have been in similar positions and if you are both serious about your relationship and settling down together then she absolutely must avoid any risk of enforcement by UKBA. This will be by either an application for Discretionary Leave to Remain (DLR) or to actually leave UK voluntarily and paying for all of the travel cost.
Without leaving the UK the only option is an application outside the immigration rules for Discretionary Leave to Remain (DLR). This would be based on you (the fiance) being in the UK.
The relevance of your girlfriend's UK residence depends on the nature of the relationship. For example, the case would be stronger if you have been cohabiting for a long time or if you can show clear evidence of a long and genuine relationship.
However, even that is still not as significant as having a spouse or a child in the UK.
Due to the discretionary nature of DLR you must do all you can to submit the strongest possible application. An experienced advisor who understands the situation in detail could reliably assess the chances of such an application.
Discretionary leave is not at all guaranteed, and can often be a very long and tedious process leaving you both living in 'limbo' and unable to implement any firm plans for the future. There may well be the need for extended representation to help with any needed follow-ups such as presentation of caselaw, negotiations with HO and even appeals etc.
Having been an overstayer is not, in itself, either a barrier or reason for refusal in applying for a UK entry visa (spouse or fiancee). However, I recall that overstaying for more than 28 days, but voluntarily leaving the UK by using your own finances (no public funds) may mean some automatic delay period before making a settlement visa application.
If I'm incorrect somebody will say so. I will try to make some time for a research of immigration rules.
But before investing significant finances in the travel and entry visa application costs, I think this option is also well worth consulting an advisor in the UK to ensure that any subsequent application for UK settlement visa has the best chance to succeed. Also to make a careful review that before returning to Phils there are no other issues arising from her period of overstay which would lead to an application from her country of citizenship being refused (though the overstay itself is not a refusal reason) based on her activities in the UK (such as illegal working aspects/not paying tax etc.or other legal declarations)
They can also do a document check and be sure that everything the ECO would expect to see is provided.
I'm not sure if this reply actually helps you at all, but at least the discussion has now started and can be moved forward.
Whatever option you finally decide upon, I wish you good luck.
