rewrite rule for .htaccess not working for php file

After few tries i found out that

RewriteEngine OnRewriteRule .* index.php

was not working but

RewriteEngine OnRewriteRule .* index.html 

was working, what could be the possible reason? and how can i fix it?

Try(loop free):

RewriteEngine On
RewriteRule ^.*$ index.php [L]

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