htaccess issue - strange [OR] behaviour - .htaccess

Question

I've noticed an issue with [OR] in htaccess that I don't understand. What I want to do is redirect every non-existent file and every file that is a .php file (and not /index.php) to /404/. By my understanding following two blocks should do the same thing:
# .htaccess 1
RewriteEngine on
# cond 1
RewriteCond %{REQUEST_FILENAME} !-f [OR]
# cond 2
RewriteCond %{REQUEST_FILENAME} !index\.php$
# cond 3
RewriteCond %{REQUEST_URI} \.php$
# rule 1
RewriteRule [^\.]*\.[a-zA-Z0-9]+$ /404/ [L]
# rule 2
RewriteRule ^(([^/\.]+)/?)*?$ /index.php [L]
and this:
# .htaccess 2
RewriteEngine on
# cond 1
RewriteCond %{REQUEST_FILENAME} !-f
# rule 1
RewriteRule [^\.]*\.[a-zA-Z0-9]+$ /404/ [L]
# cond 2
RewriteCond %{REQUEST_FILENAME} !index\.php$
# cond 3
RewriteCond %{REQUEST_URI} \.php$
# rule 2
RewriteRule [^\.]*\.[a-zA-Z0-9]+$ /404/ [L]
# rule 3
RewriteRule ^(([^/\.]+)/?)*?$ /index.php [L]
Yet, only the second block does what I want. The first .htaccess, for example - fails to redirect a non-existent file a.pdf and instead displays default server message about file not being found.
Can somebody please explain this to me?

Answer 1

Your OR conditional in the first .htaccess is in the wrong order.
The [OR] flag creates a group around the two statements, effectively writing your conditional as:
if (
(FILE_NOT_FOUND or FILE_NAME === 'index.php')
and REQUEST_URI ends in .php
) { ... }
Written in this order, the conditional will never match a URI that ends in an extension other than PHP.
What you actually want is:
if (
(FILE_NOT_FOUND or REQUEST_URI ends in .php)
and FILE_NAME !== 'index.php'
) { ... }
Which can be expressed by simply rearranging the two related rewrite conditions:
RewriteCond %{REQUEST_FILENAME} !-f [OR]
RewriteCond %{REQUEST_FILENAME} \.php$
RewriteCond %{REQUEST_URI} !index\.php
RewriteRule [^\.]*\.[a-zA-Z0-9]+$ /404/ [L]
You can even catch your mistake by reading your own words (simplified to show the actual conditional you had in mind):
What I want to do is redirect every non-existent file [or] every .php file, [and] not /index.php to /404/.
Here is a good StackOverflow question/answer about the precedence of [OR] in .htaccess files if you'd like to learn more about how mod_rewrite actually works internally.