Document USES=php.

[wblock]

en_US.ISO8859-1/books/porters-handbook/special/chapter.xml

3428 ↗

(On Diff #17807)

No comma after WWWGRP.
Most modern English speakers do not use the word "parsimony".
This might be better as two sentences:

<para>Use <varname>WWWOWN</varname> and
<varname>WWWGRP</varname> sparingly.  Remember that
every file that can be written to by the web server is a potential
security risk.</para>

3435 ↗

(On Diff #17807)

Arguably this should be s/app/application/.

Also needs an article between "on" and "different", but might be better said as

application on a web server other than Apache.</para>

3441 ↗

(On Diff #17807)

Try to avoid the if/then sentence structure with a pause in it:

<application>PHP</application> web applications declare their dependency
on it with <literal>USES=php</literal>.

3443 ↗

(On Diff #17807)

How about just

Variables are used to specify which <application>PHP</application> modules
are required.

en_US.ISO8859-1/books/porters-handbook/uses/chapter.xml

1731 ↗

(On Diff #17807)

s/informations/information/