Merging mp4 video stream with subtitles for use in creating DVD
Use the subtitles filter to create hardsubs ("burnt-in" subtitles):

ffmpeg -i input.mp4 -vf subtitles=subs.srt -target pal-dvd output.mpg



-target pal-dvd will make the output have a frame rate of 25 and a frame size of 720x576. If you want NTSC output instead for a frame rate of 30000/1001 and frame size of 720x480, then use -target ntsc-dvd. For a frame rate of 24000/1001 and 720x480 frame size use -target film-dvd.
The old, so-called "ffmpeg" version 0.8.16 in the repo is a counterfeit from a fork that lacks the subtitles filter. If you need the real thing you can simply download a static build.
I do not believe ffmpeg can convert the text-based SRT to the bitmap-based dvdsub/vobsub, but there are probably other tools that can do so. This will allow you to use standard DVD subtitles which can be turned on or off by the viewer.