Both MicroDVD (name.sub) and SubRip (name.srt) are text-based subtitles (which should be readable text if you load them e.g. in Notepad).
Often also .ssa (SubStationAlpha) and .smi (Sami) subtitles are supported. Don't know about your Philips though.
As a rule your subtitle file should have the same name as the .avi-file,
Unfortunately you do not mention which Philips DVD player it concerns.
However, maybe check out
this site (click here) for excellent third party firmware for i.a. Philips DVD-players.