Upgrading from php4 to php5

[rathersurf]

I am running PHP Version 4.3.10 ...... My question is : if i upgrade to version 5 will the scripts for 4 run in version 5...I know there are differences in scripts in different versions....I just want to make sure that if i upgrade then my current apps will still work...

Thanks in advance,

[rathersurf]

I forgot to mention that I am fairly new at php and i am developing an app. using an oracle db instead of mysql....