Tente usar o código htaccess abaixo.
# Make sure that the rewrite module has been enabled
<IfModule mod_rewrite.c>
# Enable the RewriteEngine may be duplicate.
RewriteEngine on
# Base of website
RewriteBase /
# Check if it is a PHP **file**
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.php -f
#Writing it without .php
RewriteRule ^(.*)$ $1.php
</IfModule>
Se esta for a solução, aceite minha resposta e, eventualmente, vote nela.