It should be /subdir/ since that is the directory the .htaccess is located in relative to the document root.
Well I don't have experience with 5.2.9 yet but check phpinfo() again, there is a section 'cgi-fcgi' and mine says:
cgi.redirect_status_env no value
And after moving both index.php and .htaccess to /home/username/domain/subdir/ and setting RewriteBase to /subdir/ this still works for me with 5.2.6
Try this. Just rewrite to index.php without adding the /$1
For me index.php still gets called of course but PATH_INFO and PATH_TRANSLATED are not set, so there won't be a value of '/$1' set anywhere. BTW if you are just trying to make search engine friendly URLs just rewrite to a query string, ie index.php?path=$1. Seems like getting PATH_INFO to work as expected is harder for PHP to do than one expects. Don't have this much trouble in with Perl at least.
Oh yeah don't forget if you have .htaccess in the parent directory then that will affect /subdir too so you might be seeing a side effect.
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