Migrating from PHP 4 to PHP 5.0.x
PHP Manual

Databases

There were some changes in PHP 5 regarding databases (MySQL and SQLite).

In PHP 5 the MySQL client libraries are not bundled, because of license and maintenance problems. MySQL is supported with the only change being that MySQL support is no longer enabled by default in PHP 5. This essentially means that PHP doesn't include the --with-mysql option in the configure line so that you must now manually do this when compiling PHP. Windows users will need to edit php.ini and enable the php_mysql.dll DLL as in PHP 4 no such DLL existed, it was simply built into your Windows PHP binaries.

There is also a new extension, MySQLi (Improved MySQL), which is designed to work with MySQL 4.1 and above.

Since PHP 5, the SQLite extension is built-in PHP. SQLite is an embeddable SQL database engine and is not a client library used to connect to a big database server (like MySQL or PostgreSQL). The SQLite library reads and writes directly to and from the database files on disk.


Migrating from PHP 4 to PHP 5.0.x
PHP Manual