These functions allow you to access Oracle Database 12c, 11g, 10g, 9i and 8i. They support SQL and PL/SQL statements. Basic features include transaction control, binding of PHP variables to Oracle placeholders, and support for large object (LOB) types and collections. Oracle's scalability features such as Database Resident Connection Pooling (DRCP) and result caching are also supported.
There are several good books on PHP and Oracle available - search your favorite bookstore. There is also a free book from Oracle "The Underground PHP and Oracle Manual" that covers the OCI8 extension:
http://www.oracle.com/technetwork/topics/php/underground-php-oracle-manual-098250.html
(free registration for OTN required, IIRC). Disclaimer: I'm one of its authors.