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If wordpress cannot append .php, then is it possible to add something to htaccess to let all the old urls continue to be used?
If you have a bunch of names getting changed in the same way, it would be
RewriteRule ^pagename\.php /pagename/ [L]
These new, rewritten URLs will in turn be passed along to WP, which will handle them just as if the user had requested the /pagename/ form in the first place.
RewriteRule ^(onepage|otherpage|thirdpage)\.php /$1/ [L]
RewriteRule ^widget/([^/\.]+).php$ widget/$1/
though personally I like
at a savings of three bytes, assuming the URLs don't contain hyphens. (Possibly they do, since WP does love its hyphens.)
RewriteRule ^widget/([^/.]+)\.php$ /widget/$1/ [L]
Check one of the radio buttons corresponding to the correct Permalink Structure for your blog.
Default - An example of the default structure is http://www.sample.com/?p=123
Day and name - An example of the day and name based structure is http://www.sample.com/2008/03/31/sample-post/
Month and name - An example of the month and name based structure is http://www.sample.com/2008/03/sample-post/
Numeric - An example of the numeric structure is http://www.sample.com/archives/123
Post name - An example of the post name structure is http://www.sample.com/sample-post
Custom structure - In the box specify the custom structure you desire to use. One example is /archives/%year%/%monthnum%/%day%/%postname%/. Look at the Using Permalinks article for further discussion of Permalink Structure Tags.