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For documentation on the legacy kernel, please go to doc.ez.no where you'll find documentation on everything from it's its GUIs, official extensions to template coding ( 5.x doc: User manual & Technical manual ). If you are looking for documentation on how to develop extensions for legacy in php, then also see relevant material on share.ez.no ( Tutorials, Blogs ).
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For documentation on Symfony2 go to symfony.com (for Twig see here) and for the eZ Publish parts of 5.x kernel continue reading in this section of our new wiki documentation area.