1.
Migrating from PHP 3 to PHP 4
PHP 4 was designed to be as compatible with earlier versions of
PHP as possible and very little functionality was broken in the
process. If you're really unsure about compatibility you should
install PHP 4 in a test environment and run your scripts there.
Also see the appropriate migration
appendix of this manual.
2.
Do sessions work in PHP 3?
Although native session support
didn't exist in PHP 3, there are third-party applications that did
(and still do) offer session functionality. The most common method
was by using PHPLIB.
3.
Incompatible functions?
Since PHP 4 is basically a rewrite of the entire PHP engine there
were very few functions that were altered and only then some of
the more exotic ones.
Migrating from PHP 2 to PHP 3