newbie migration issues

Hi, all - Hoping someone can help me with this..I have a client who has a webiste with a number of php3 scripts in it, and we're having a spate of problems relating to migration to php4. 1) I need to know what needs to be done to convert php3 scripts to be compatible with php4. These are scripts largely to do with counters, statistics, date & time functions, and the like. What needs to be done to their extensions as well? 2) Not having access to that info at the time, we decided to go back to php3 - and then ran into a host of security issues re visibility of the html directory. The requisite libraries were there, apache was configured to work with php3, and so forth. If anyone can explain how security issues arose, and give me some faint clue as to how to prevent that from ever happening again,I will try to send maple syrup. 8) 3) Can anyone begin to explain the rationale for making php3 and php4 so utterly incompatible? Thanks in advance, Alpha