Where the headers are?

Hi! I'm a bit new to libmemcache. I have it installed on Ubuntu through the libmemcached5 package (i also installed the libmemcached-dev). I'm trying to compile the PHP extension memcached, which uses libmemcached. For compiling the extension, i have to add the following parameter:

--with-libmemcached-dir=[libmemcache dir]

I tried with /usr/include/libmemcached/ /usr/lib, etc and I always get the error Can't find libmemcached headers under "[the path i wrote]"

Where the headers are located? Thank you