When I enable the mod_throttle module on Apache my PHP pages stop
working. Sometimes I can see the PHP source instead of a generated
page. I'm running RedHat Linux 7.1 with mod_throttle 3.1.2 compiled
from source.

Name        : apache
Version     : 1.3.27 
Release     : 1.7.1 

Name        : php
Version     : 4.1.2 
Release     : 7.1.6

mod_throttle and mod_php can peacefully co-exist. the problem could
possibly be due to the mod_throttle installation overwriting/disabling
the lines in httpd.conf that specify the php handler and index types.
make sure that after installing both apache modules, your httpd.conf
still includes lines similar to:

LoadModule php4_module        libexec/apache/libphp4.so 
AddModule mod_php4.c

<IfModule mod_dir.c>
    <IfModule mod_php3.c>
        <IfModule mod_php4.c>
            DirectoryIndex index.php index.php3 index.html
        </IfModule>
        <IfModule !mod_php4.c>
            DirectoryIndex index.php3 index.html
        </IfModule>
    </IfModule>
    <IfModule !mod_php3.c>
        <IfModule mod_php4.c>
            DirectoryIndex index.php index.html
        </IfModule>
        <IfModule !mod_php4.c>
            DirectoryIndex index.html
        </IfModule>
    </IfModule>
</IfModule>

PHP worked fine again after disabling the mod_throttle lines in
httpd.conf so I don't think any lines were overwritten or disabled.