rewrite (.html <-> .htm) -> .php -> root

rewrite-extenison

# rewrite .html to .htm only if .html does not exist and .htm exist
RewriteCond %{REQUEST_FILENAME} (.*)\.html$
RewriteCond %1\.html !-f
RewriteCond %1\.htm -f
RewriteRule ^(.*)\.html$ $1.htm [L]

# rewrite .htm to .html only if .htm does not exist and .html exist
RewriteCond %{REQUEST_FILENAME} (.*)\.htm$
RewriteCond %1\.htm !-f
RewriteCond %1\.html -f
RewriteRule ^(.*)\.htm$ $1.html [L]

# rewrite .html to .php only if .html does not exist and .php exist
RewriteCond %{REQUEST_FILENAME} (.*)\.html$
RewriteCond %1\.html !-f
RewriteCond %1\.php -f
RewriteRule ^(.*)\.html$ $1.php [L]

# rewrite .htm to .php only if .htm does not exist and .php exist
RewriteCond %{REQUEST_FILENAME} (.*)\.htm$
RewriteCond %1\.htm !-f
RewriteCond %1\.php -f
RewriteRule ^(.*)\.htm$ $1.php [L]

# redirect to homepage if file does not exist and is not under blog/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !blog/
RewriteRule . / [L,R=301]