comments on methods of developing PHP applications

Hi to all php developers ! My question is not a technical one, but on the strategy of developing php applications. i've been learning/writing php codes since 6-7 months and now my php applications start to get more and more complex and i haven't yet found a successful method for developing php programs efficiently, how do you start it ? do you 1st make the HTML user interface 1st and then fill-in the php code or do you 1st write the code and then write the HTML interface and where does the SQL part come ? i'm in the process of writing an application with more than 5000 lines of php code and i don't know where to start ? should i do 1st the GUI and then the php code and sql staff or what ? i know this changes from one person to another, but i'd really appreciate any feedbacks because there are so many things to do at once that i feel like drowning (so many input field names to remember, sql table structures to do, etc...) Irfan Uygur