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I installed php-manual, which installs itself into the Apache document tree, and discovered that my root paritition (which includes /var) all but filled up. I think this violates the principle of least surprise. The Apache documentation is not variable data, so it should be in /usr/share/doc. When installing documentation, one expects it to go into /usr. The default Apache installation should symlink from the HTML document directory (/var/www/html) to this. Separate module packages (such as PHP) should follow this model and keep their documentation in /usr/share/doc/<package>, symlinked from the appropriate subdirectory of the Apache documentation.
Thanks for the report. php-manual is no longer included in Fedora Core or Red Hat Enterprise Linux, which should avoid this particular issue. /var/www is the place where all web content is stored by default, so is appropriate as the location of the manuals.