PHP Files and 404/.htaccess Problem

A user on another (non-WordPress-related) forum posted about an error he (or she) was having where he created a custom 404 error page and pointed to it with the .htaccess file. His problem was, while the error file came up fine if a user typed in a path to a false .htm file, a different page came up for false .php files, a white page that only had the text, “No input file specified.” I found that this was the case for me, too: Typing in “http://www.sassydevil.com/blue.htm” or “http://www.sassydevil.com/blue” brings up the correct file, but “http://www.sassydevil.com/blue.php” does not.

I tried the solution in the forum, to put the code below into the .htaccess file, but it didn’t work for me:

RewriteEngine On
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteRule (.+) /404.html

I even tried changing the “404.html” to “404.php,” since my 404 file is a php file. I tried putting this below the code I have below the WordPress rewrite stuff, which is this:

# END WordPress

ErrorDocument 404 /index.php?error=404
ErrorDocument 403 /index.php?error=403
Options All -Indexes

Can someone tell me what I need to do? Thanks.