The PHP/Java bridge allows you to quickly access Java classes from within your PHP classes without having to know Java and vice versa. Because of this two-way flexibility, you can access hundreds of pre-built Java classes from PHP, and hundreds of pre-built PHP classes from Java opening up your applications to greater flexibility and enhanced functionality.

The post includes an introduction to the bridge's functionality - its functions/methods, how to make a new instance of it, catching exceptions and the type conversion it automatically does (like lang.java.String directly to the PHP String type).

They also mention the Zend Platform bridge functionality for a more commercial solution.