$ sudo port install php5-mysql
Password:
--->  Computing dependencies for php5-mysql
--->  Fetching php5-mysql
--->  Verifying checksum(s) for php5-mysql
--->  Extracting php5-mysql
--->  Configuring php5-mysql
--->  Building php5-mysql
--->  Staging php5-mysql into destroot
--->  Installing php5-mysql @5.3.0_0+mysqlnd
To use mysqlnd with a local MySQL server, edit /opt/local/etc/php5/php.ini and set
mysql.default_socket, mysqli.default_socket and pdo_mysql.default_socket
to /opt/local/var/run/mysql5/mysqld.sock
--->  Activating php5-mysql @5.3.0_0+mysqlnd
--->  Cleaning php5-mysql

　表示されているメッセージにあるように、php.iniにdefault_socketを設定するのを忘れないこと。

To use mysqlnd with a local MySQL server, edit /opt/local/etc/php5/php.ini and set
mysql.default_socket, mysqli.default_socket and pdo_mysql.default_socket
to /opt/local/var/run/mysql5/mysqld.sock