After recently upgrading php5 in Debian Sid, I started getting this problem (in phpBB3 and MantisBT, etc):
PHP Fatal error: PCRE is not compiled with UTF-8 support in (...)
It seems the cause was wrong version of libicu44. Upgrading it as well fixed the problem.
This old tip of mine should fix cases like this with outdated dependencies:
/2010/06/04/how-to-upgrade-all-dependencies-of-a-debian-package/
/2010/06/04/how-to-upgrade-all-dependencies-of-a-debian-package/