PHP Startup: Unable to load memcache.so

I noticed the warning
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/lib/php/20090626/memcache.so' - /usr/local/lib/php/20090626/memcache.so: Undefined symbol "php_session_create_id" in Unknown on line 0

in my apache logs. Googling for it, some sites suggested that old php.in files might the cause. But this is a pretty fresh install so I looked a little further and found a nerdstock note describing a similar problem. I checked my /usr/local/etc/php/extensions.ini and finding that extension=session.so came after extension=memcache.so I moved it to the top of the file and the errors went away.