There appears to be a bug with the newest version of PHP and it's associated packages from DotDeb and the latest version of Apache 2.4 available on Mint.
PHP 5.5.9, available through the default Mint packages works fine. No errors, and I can get phpinfo() to display results properly in the browser. When I upgrade to DotDeb's latest version (5.5.16) and it's associated Apache2 package upgrades, I get the following upon server restart:
$ sudo /etc/init.d/apache2 restart
* Restarting web server apache2 [fail]
* The apache2 configtest failed.
Output of config test was:
apache2: Syntax error on line 140 of /etc/apache2/apache2.conf: Syntax error on line 1 of /etc/apache2/mods-enabled/php5.load: Cannot load /usr/lib/apache2/modules/libphp5.so into server: /usr/lib/apache2/modules/libphp5.so: undefined symbol: unixd_config
Action 'configtest' failed.
The Apache error log may have more information.
The error log doesn't shed any light on it, despite the suggestion from the error output. Also note that nothing has changed except for me upgrading PHP versions and the Apache packages that are automatically included.
Looking for a solution elsewhere, I found the following which makes it look like a known bug (here and here).
Given that Mint is always a release or two behind Ubuntu, is there a way for me to upgrade Apache 2 itself to the latest version that, presumably, has the bug fix? A package repo to point to, or something along those lines?
Further research and discussions with colleagues have made it clear that dotdeb's version of PHP has been compiled against Apache 2.2. Apparently version 2.4.x doesn't use the unixd_config symbol.
EDIT: One of my colleagues showed me this PPA which did the trick. I'm now updated with the latest versions of Apache 2.4 and PHP 5.