I found PHP on CentOS 6 built with mysql.default_socket=/var/lib/mysql/mysql.sock yet for some reason pdo_mysql.default_socket=/tmp/mysql.sock, which doesn't exist. Building it using --with-mysql-sock=/var/lib/mysql/mysql.sock should force both to be the same.
Alternatively edit php.ini to contain:
pdo_mysql.default_socket="/var/lib/mysql/mysql.sock"
...or your correct socket location, of course.
