The online manual is an excellent resource for the language syntax and has an extensive list of the built-in and extension functions. Most extensions can be found in PECL. PEAR contains a plethora of community supplied classes.
PHP is often paired with the MySQL relational database. PHP also includes great database support for PostgreSQL, SQLite, Microsoft SQL Server (API ref), Oracle, IBM DB2 & Cloudscape, Apache Derby and even ODBC. All modern versions of PHP include PDO, a built-in data-access abstraction library with comprehensive connectivity options.
Useful Third-party Code
PHP has a variety of object-oriented web application frameworks, including:
Popular ORMs for PHP include:
(See: "Good PHP ORM Library?")
Popular unit testing frameworks for PHP include: