How do you create an image of a web page in PHP?

How can you render and save a web page as an image in PHP, probably with a width of 600px. How can I render a page in PHP without using a browser? How can I save it with a given resolution and image format (jpeg)? The functionality is similar to Google Preview, except it will not be displayed in a rollover.

Similar to this question, which is answered in C#. How to save a web page as image

Thanks!