PDO and the PDO_SQLITE driver is enabled by default as of PHP 5.1.0.
You may need to enable the PDO driver for your database of choice;
consult the documentation for database-specific PDO drivers to find out more about
that.

Note:

When building PDO as a shared extension (not recommended) then all PDO drivers must be loaded after PDO
itself.

When installing PDO as a shared module, the php.ini
file needs to be updated so that the PDO extension will be loaded
automatically when PHP runs. You will also need to enable any
database specific drivers there too; make sure that they are listed
after the pdo.so line, as PDO must be initialized before the
database-specific extensions can be loaded. If you built PDO and
the database-specific extensions statically, you can skip this
step.

extension=pdo.so

PDO and all the major drivers ship with PHP as
shared extensions, and simply need to be activated by editing the
php.ini file:

extension=php_pdo.dll

Note:

This step is not necessary for PHP 5.3 and above,
as a DLL is no longer required for PDO.

Next, choose the other database-specific DLL files
and either use dl() to load them at runtime, or enable
them in php.ini below php_pdo.dll. For example:

extension=php_pdo.dll
extension=php_pdo_firebird.dll
extension=php_pdo_informix.dll
extension=php_pdo_mssql.dll
extension=php_pdo_mysql.dll
extension=php_pdo_oci.dll
extension=php_pdo_oci8.dll
extension=php_pdo_odbc.dll
extension=php_pdo_pgsql.dll
extension=php_pdo_sqlite.dll  

These DLLs should exist in the system’s extension_dir.