Mail_Queue PHP Crond - mail sends but PHP process hangs

Question

This has been causing me a problem since I upgraded to PHP 5.3. The problem didn't happen with PHP 5.2.

I have a PHP script (which is pretty much the stock Mail_Queue script) to send messages.

I'm executing the script via cron:

php /home/public_html/send_messages.php

Email is successfully sent, however I get the PHP process hanging, along with the cron and postfix. Killing the PHP process solves the problem.

I get this output when executing the cron job (via webmin):

PHP Notice:  Error in sending mail: Mail Queue Error: Cannot initialize container in /usr/share/pear/PEAR.php on line 873

When I execute the PHP script from a browser this problem doesn't happen.