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create a htaccess file in the root folder of your web directory. And have to put the following codes as your requirement.
Options +FollowSymlinks
RewriteEngine on
RewriteRule ^(.*)\.htm$ $1.php [nc]
Example
test.php to test.htm
http://localhost/test.htm
URL rewrite
Options +FollowSymlinks
RewriteEngine on
RewriteRule ^product-([0-9]+)\.html$ products.php?id=$1
example
rewrite the product.php?id=5 to porduct-5.html
http://localhost/product-5.html
calls product.php?id=5