Are there any tools or techniques for php to discover all classes particular class depends on ? For example to find out which classes CActiveRecord (Yii framework) class depends on (CComponent, CDbException, CHasOneRelation and so on)
You can use the ReflectionClass to find out the dependencies, in particular the getParentClass() method provided.
http://www.php.net/manual/en/reflectionclass.getparentclass.php
You could get your hands dirty and write some code to utilise ReflectionClass - or run phpdoc http://www.phpdoc.org/ on your codebase and then read the generated documents.