.htaccess redirect html to php and access to php files without .php extension

Try something like this. Follow the comment thread for security implications, but this is what you're asking to do

RewriteEngine On
RewriteBase /

# Redirect HTML to PHP
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*).html$ $1.php [L]

# Otherwise, try PHP
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !\.php$
RewriteRule ^(.*)$ $1.php [L]

# Lastly, fallback to error page
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . 404.php [L]