This is more a PHP and FreeBSD problem than SquirrelMail, but I believe it may be relevant here as other admins may experience the same problem, and may instinctively search here first as SquirrelMail is one of the most affected apps. Also note that PHP 4.3.10 was a security upgrade. If you do have any problems upgrading to it, please read on.
After an upgrade to PHP 5 on a FreeBSD system the SquirrelMail front page to show up blank. Nothing appears amiss in the Apache log files, but there is no output.
Verify your PHP extensions have all been recompiled against the current libraries. Turn up debug logging in PHP and verify that all of the extensions listed in /usr/local/etc/php/extensions.ini have been recompiled.
On a FreeBSD 4.11 system, an upgrade of PHP was done before the installation of OSCommerce. PHP 4.3.10 somehow caused a the SquirrelMail front page to show up blank, with only the <html> and <body> tags present when you view the source. When you view the web server's log files, Apache's child processes were killed.
I spent numerous hours searching Google and trying to update PHP, MySQL and Apache on this system in an attempt to solve the problem. I had then viewed a post from [firstname.lastname@example.org] which related to my problem.
Downgrade to PHP 4.3.9, either by using the portdowngrade and portupgrade ports, or by downloading and installing it manually. Here are the steps when using the first method:
portdowngrade -o lang/php4
(theoretically the following two lines should work, but they did not for me. in that case, just cd /usr/ports/lang/php4; make deinstall; make install clean)
portupgrade -f php4