If you want to avoid installing DB2 Connect and/or PECL modules ibm_db2 and PDO_IBM, you can also use IBM DB2 databases trough unixODBC.
If you have DB2 database on a i server you need to install IBM iAccess (http://www.ibm.com/systems/i/software/access/linux/index.html) and unixODBC. Just install the libraries (rpm) and modify configurations in /etc/odbcinst.ini (sample configuration in /opt/ibm/iSeriesAccess/unixodbcregistration) and /etc/odbc.ini.
To my experience this is much easier way than installing DB2 Connect.