"It's no secret that PHP has changed significantly since the
earliest versions of PHP 4 were released almost a decade ago.
Indeed, each major revision of PHP has required code changes, as
language features were added, modified, and obsoleted. Depending on
your PHP code, PHP 6 will be no exception.
"Although PHP 6 isn't yet available as a pre-built package, you
can download and install a development snapshot of PHP 6 now to
check out the new features and verify your scripts remain
functional. Since PHP 6 removes some backwards-compatibility
features (which, in the long term, is a good thing), you should
test your existing code thoroughly."