See the sidebar, then on the download page, look for the links above the table. Maybe we need big download buttons like firefox for people who are not reading the page text.

Finally I have found it. But not by the assist of your description, but rather from the page source of this page:
http://www.php.net/download-docs.php
I follow all the link one by one and finally found out there is a difference between 
http://www.php.net/get/php_manual_chm.zip/from/a/mirror
and
http://www.php.net/get/php_manual_en.chm/from/a/mirror
which only a single word of 'chm.zip' and 'en.chm', which is really very hard to find out.

My advise, as being a maintainer of different website for many years, users won't be able to find out that easily if you put the link on a word, which is the 'downloadable' here. I have asked all of my friend to find this out but nobody could. All will go to the table which put together all languagues. 

Suggested to put that extended chm link into the table where there are the different language of chm in this page:
http://www.php.net/download-docs.php

Or may be a simple icon in this page:
http://www.php.net/docs-echm.php
cause people will first find the link in this description page. I have tried so hard but just could get a link in this page.

I never say it's wrong but just may be a clearer way for people to find the link. And next time please don't put assumption that 'people didn't read the page text', as that is being rude. Thanks.