automatically updating software

Hi, I want to write some (browser based) software that will get installed on the client's servers, and I want it to update automatically (as in Windows XP: new updates are downloaded and presented for install). Can that be done with PHP? Some ideas so far (that will probably betray my ignorance :): - updating images will require write access that may introduce security issues? - keeping almost all code in a database and update that? - or require write access to a directory outside of the web root? - any existing approaches - apps already doing it? - what are the limits of updates distributed this way? - how much extra coding does this method imply? Peter