Even though I'm on a linux machine now, you 'should' be able to recompile php with the settings plesk has by default, and add your features, without breaking plesk... although this is definitely untested, as you've found out:
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[root@plesklinux root]# php -i | grep configure
<tr><td class="e">Configure Command </td><td class="v"> './configure' '--host=i386-redhat-linux' '--build=i386-redhat-linux' '--target=i386-redhat-linux-gnu' '--program-prefix=' '--prefix=/usr' '--exec-prefix=/usr' '--bindir=/usr/bin' '--sbindir=/usr/sbin' '--sysconfdir=/etc' '--datadir=/usr/share' '--includedir=/usr/include' '--libdir=/usr/lib' '--libexecdir=/usr/libexec' '--localstatedir=/var' '--sharedstatedir=/usr/com' '--mandir=/usr/share/man' '--infodir=/usr/share/info' '--cache-file=../config.cache' '--with-config-file-path=/etc' '--with-config-file-scan-dir=/etc/php.d' '--enable-force-cgi-redirect' '--disable-debug' '--enable-pic' '--disable-rpath' '--enable-inline-optimization' '--with-bz2' '--with-db4=/usr' '--with-curl' '--with-dom=/usr' '--with-exec-dir=/usr/bin' '--with-freetype-dir=/usr' '--with-png-dir=/usr' '--with-gd' '--enable-gd-native-ttf' '--with-ttf' '--with-gettext' '--with-ncurses' '--with-gmp' '--with-iconv' '--with-jpeg-dir=/usr' '--with-openssl' '--with-png' '--with-pspell' '--with-regex=system' '--with-xml' '--with-expat-dir=/usr' '--with-pcre-regex=/usr' '--with-zlib' '--with-layout=GNU' '--enable-bcmath' '--enable-exif' '--enable-ftp' '--enable-magic-quotes' '--enable-safe-mode' '--enable-sockets' '--enable-sysvsem' '--enable-sysvshm' '--enable-discard-path' '--enable-track-vars' '--enable-trans-sid' '--enable-yp' '--enable-wddx' '--enable-mbstring' '--enable-mbstr-enc-trans' '--enable-mbregex' '--without-oci8' '--with-pear=/usr/share/pear' '--with-imap=shared' '--with-imap-ssl' '--with-kerberos=/usr/kerberos' '--with-ldap=shared' '--with-mysql=shared,/usr' '--with-pgsql=shared' '--with-unixODBC=shared' '--enable-memory-limit' '--enable-bcmath' '--enable-shmop' '--enable-versioning' '--enable-calendar' '--enable-dbx' '--enable-dio' '--enable-mcal''--enable-force-cgi-redirect' </td></tr>
[root@plesklinux root]#
You should be able to run that on your bsd machine, and see how php was compiled. Plesk keeps its own version of php for the admin interface, so you shouldn't break that with the install.
running the normal:
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./configure "with above options plus yours"
make && make install
It 'may' not destroy your current setup, although I do wonder about your libphp4.so that apache uses, as I don't see the apxs option for the php configure...which is odd.... Please take this as experimental.