There are two main (and different) methods for checking out the PHP documentation sources.
Regardless, the following descriptions will setup a structure like the following:
phpdoc/en/ phpdoc/doc-base/
Where en/ contains all the XML DocBook files specific to English, and doc-base/ contains the related helper files like PHP scripts for validation.
These examples are specific to English (en), so substitute with the appropriate language code to instead download a translations sources.
Several svn:externals already exist within phpdoc/modules and may be used. They are setup as doc-{lang} where {lang} is the language code for the desired language, and doc-all exists to include every language.
svn co https://svn.php.net/repository/phpdoc/modules/doc-en phpdoc
So, that checks out via the svn:externals to a new local directory named phpdoc.
Sparse checkouts are another option.
svn co https://svn.php.net/repository --depth empty phpdoc svn co https://svn.php.net/repository/phpdoc/en/trunk --depth infinity phpdoc/en svn co https://svn.php.net/repository/phpdoc/doc-base/trunk --depth infinity phpdoc/doc-base
A directory named 'phpdoc' (although you can change this, but the above uses it) will contain the two directories (en/ and doc-base/) so a test might be:
cd phpdoc php doc-base/configure.php
That'll configure and validate the checkout.
The above instructions (for svn:externals and sparse checkouts) checkout trunk from SVN. This is because the manual rarely uses branches or tags, but checking out via one of the above methods is certainly not required, but rather is meant to keep things simple.