Split Editor
The Split Editor in Zend Studio allows viewing the same class or file side by side. Using the split editor, you can for example follow related code in different parts of the class, or have code constantly available as reference to change some other part of the class.
Zend Studio allows you to split an editor horizontally or vertically and move it to anywhere in the editor area in the same window, including next to the original editor. Changes made in the one editor are reflected in the other.