In the previous section, we learned
about the placeholder() view helper, and how it can be used to
aggregate custom content. In this section, we'll look at some of the concrete placeholders
shipped with Zend Framework, and how you can use them to your advantage when creating
complex composite layouts.
Most of the shipped placeholders are for generating content for the <head> section of your layout content -- an area you typically cannot manipulate directly via your application view scripts, but one you may want to influence. As examples: you may want your title to contain certain content on every page, but specific content based on the controller and/or action; you may want to specify CSS files to load based on what section of the application you're in; you may need specific JavaScript scripts loaded at different times; or you may want to set the DocType declaration.
Zend Framework ships with placeholder implementations for each of these situations, and several more.