Just like ADOdb, it enables the PHP application to be written once and run with 

minor changes for various database engines such as MySQL, Oracle, 

PostgreSQL, or DB2 UDB.

To provide database abstraction features, PEAR DB uses an object oriented 

methodology such as classes and objects. For doing so it uses three files:

DB.php

This class contains implementation of the DB class, which creates database 

connection objects.

common.php

This class contains generic code for all the database specific implementation 

of the database access methods. All the drivers inherit and overload this 

code.

.php

This file contains database specific implementation of the access code. This 

class is also called the driver class and is used by PHP applications. In case 

of MySQL, the driver file is 

mysql.php,

 whereas in the case of DB2 it is 

odbc.php

.

The way PEAR DB provides its services for a DB2 UDB application and MySQL 

application is quite different. In MySQL PEAR DB uses the currently existing 

extension for MySQL database, whereas PEAR DB does not provide any 

extension for connecting to DB2 UDB, so the ODBC extension is used for the 

DB2 UDB connection.

The PHP application that uses PEAR DB for database connection generally 

performs the following steps:

Include PEAR DB objects

As discussed before PEAR DB requires three files 

DB.php

, 

common.php

 and 

.php

 for proper functioning of the PHP application. But you 

need not explicitly include all these files, you just need to include 

DB.php

 and 

its functions will include the required driver depending upon your data source 

name.

So for both MySQL and DB2 UDB the include statements will be same as:

include( /home/itso/php/PEAR/DB.php );

Connect to database

PEAR DB database connection can be established using connect method of 

declared in 

DB.php

. This method requires data source name (DSN) 

containing in connection parameters as input. The DSN is a URL style string 

that consists of the database driver name, the user name, the password, the 

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MySQL to DB2 UDB Conversion Guide