1.
Migrating from PHP3 to PHP4
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.
Incompatible functions?
Since PHP 4 is basically a rewrite of the entire PHP engine there
was very few functions that were altered and only then some of
the more exotic ones.
Migrating from PHP 2 to PHP 3
