Well firstly you would need to rip the dvds to computer which isnt legal in many cases, nor is downloading copyrighted material, then you would need to convert and compress the video for iphone compatibility.
Of course if you search hard enough you will find the stuff you want already converted, or a guide how to do so.
You are legally aloud to rip all your dvds for personal use iirc
In the United Kingdom, making a private copy of copyrighted media without the copyright owner's consent is illegal: this includes ripping music from a CD to a computer or digital music player. The UK government has made proposals to allow people to make copies of music for personal use. According to one survey, 55% of British consumers believed ripping a CD to be legal, and 59% admitted to doing it.