I installed according to the directions and now get:
Warning: require_once(/home/herbal7/public_html/mwiki/extensions/PageInCat/PageInCat.php): failed to open stream: No such file or directory in/home/herbal7/public_html/mwiki/LocalSettings.php on line 217
Fatal error: require_once(): Failed opening required '/home/herbal7/public_html/mwiki/extensions/PageInCat/PageInCat.php' (include_path='/home/herbal7/public_html/mwiki/vendor/pear/pear_exception:/home/herbal7/public_html/mwiki/vendor/pear/console_getopt:/home/herbal7/public_html/mwiki/vendor/pear/pear-core-minimal/src:/home/herbal7/public_html/mwiki/vendor/pear/mail_mime:/home/herbal7/public_html/mwiki/vendor/pear/mail_mime-decode:/home/herbal7/public_html/mwiki/vendor/pear/net_socket:/home/herbal7/public_html/mwiki/vendor/pear/net_smtp:/home/herbal7/public_html/mwiki/vendor/pear/mail:.:/usr/local/lib/php') in /home/herbal7/public_html/mwiki/LocalSettings.php on line 217
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I notice that there's no PageInCat.php in the extensions/PageInCat directory, so obviously this is the problem. I downloaded from three places, and it's not there in any of them. If I changed the name to PageInCat_body.php in LocalSettings.php, the above error goes away, but the #incat code doesn't work (I see the curly braces and everything.)