Oracle Database 11g Release 2 Express Edition aka "Oracle XE" is now available on Linux 64 bit and Windows 32 bit. This is a free version of the Oracle Database. Windows 64 bit is "in the works" but Linux 32 bit is not planned.
Check out the newly updated Oracle Database Express Edition 2 Day + PHP Developer's Guide. As well as HTML and PDF variants, the manual is available in Mobi and EPUB formats.
The 2 Day + PHP manual has steps for installing PHP and walks through creating an introductory application. It shows different ways to interact with Oracle XE and introduces PHP 5.3 features.
The example shows the mechanics of DB interaction. It builds the application from the ground up so you can understand how to construct your own high performance applications. If you want to continue the learning path and use a PHP framework, Oracle 11g XE works with the Developing a PHP Web Application with Oracle Database 11g training.
The Oracle 11.2 XE Linux install is RPM based. It takes just a few minutes to install, prompting only for a few necessary details such as desired passwords and ports. You need root access to install it.
Oracle 11.2 XE supports DRCP connection pooling so you'll really be able to maximize the small footprint database for PHP applications.
Readers who want to dive deeper into detail about PHP and advanced features available in other editions of Oracle Database might be interested in the later sections of the Underground PHP and Oracle Manual.