I ran into a fun one just now. I had compiled PHP and memcached properly, adding the extension="memcached.so" configuration line to the php config file, but it wouldn't load.
The problem? My syntax. While the correct format in the php.ini file is:
extension="memcached.so"
I had:
extension=”memcached.so”
See the difference? I didn't for a while. It's a quote-entity versus a right-double quotation mark. Killer to tell as they look really similar in the config file. Careful with those ones.