WebCalendar is a PHP-based calendar application that can be configured as a single-user calendar, a multi-user calendar for groups of users, or as an event calendar viewable by visitors.

phpEventCalendar is a MySQL backed application that allows users to post and display events or notes on a month-at-a-glance calendar.

Monket Calendar is an Ajax enabled online calendar. Drag and drop events to change dates, drag the start/end of an event to create multi-day events, create and edit events without refreshing the page, all with an iCal style interface.

PHP iCalendar is a PHP-based iCal file viewer/parser to display iCals in a Web browser.

Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL. It has been developed with ease of use and quick access to information in mind.

Here's a very simple, but very complete, PHP script to print out a calendar.

LWC is a light weight calendar written in PHP using MySQL. Easily installable. Simplistic single click JavaScriopt empowered user interface. Customizable php code for easy integration in a larger context.

ACal is a PHP5 web based event calendar designed for ease-of-use.