How do I redirect .php pages to .html pages? [duplicate]

You'll need a couple rewrite rules to do that:

#Redirects if a request for a php files comes in.
RewriteCond %{REQUEST_URI} .php$
RewriteRule ^(.*)\.php.?$ $1.html [R=301,L]

#Rewrites requests for html files to find the php file on disk.
RewriteRule ^(.*)\.html$ $1.php