RewriteEngine on
RewriteRule ^(.*)\.html$ $1.php%{QUERY_STRING} [L]
Put these two lines at the top of your .htaccess file. It will show .html in the URL for your .php pages.
RewriteEngine on
RewriteRule ^(.*)\.php$ $1.html%{QUERY_STRING} [L]
Use this for showing .php in URL for your .html pages.