seperating php code from html?

hi... i heard that it was better if when writing php to keep as much of the php code away from the html source as possible... or at least have as little html code in the php scripts as possible.. but either way it goes was just told to keep the 2 apart from each other as much as possible.. so given that fact 1. is that a true statement i should try to follow? or doesnt it really matter 2. is it ok to write code that would maybe take up 15 lines or so and turn it into 1 or 2 lines? 3. is code better written where the person who wrote it and everyone else can read it or should it be written for speed .. and last 4. when writing a class like a db class is it normal to be able to use 1 line of code after writing it instead of 6 before it was writen.. (guess this sort of fits in with the first thing but just making sure)...