The Windows HTML Help Edition of the PHP Manual is aimed at developers, who want the manual at hand every time, and would like to read manual pages and user notes without waiting for a remote web page to download. This edition gives you the full content of the manual plus the user notes, with some great features provided by the viewer application by the scripts we use to generate this format and by other contributors in the form of skins.
This distribution of the PHP Manual contains many files, including the main manual file php_manual_en.chm. You can view the manual by double clicking on this file (as you already guessed). The user notes are in php_manual_notes.chm. You only need to have one copy of this file on your computer, regardless of how many languages you use to read the PHP Manual. The only requirement is that you need to have all php_manual_LANG.chm files in one directory with php_manual_notes.chm. You may also see a php_manual_en.chw file in the directory, it's generated the first time you use the index tab, and it contains an index with the manual's pages.
For those interested in technical details about Microsoft HTML Help, » http://www.helpware.net/ is a great resource. For more information and positioning of Windows HTML Help in the Microsoft help formats and services history, see this » WinWriters article.