freeze PHP5 at specific version

[Doug Sampson]

Recently I discovered that PHP 5.3.x broke my Viart installation. It turned
out that ZendOptimizer 3.3.0.a cannot function with PHP 5.3.x. Thus I was
forced to downgrade down to PHP5-5.2.12_2.

Question: How can I force portupgrade not to upgrade to PHP5-5.3.x? I've
tried googling and researching ports management manpages to no avail. I
noticed that there's a line for Perl in /etc/make.conf that freezes Perl at
a specific version (i.e. PERL_VERSION=5.10.1) but I do not see anything
similar to that for PHP5. I must be missing something! I do not know the
appropriate search keyword for freezing a port at a specific version.

~Doug