Improve user experience when extension files not found

Description

(Context: User having issues enabling extensions on PHP on Windows: https://www.reddit.com/r/PHPhelp/comments/1mrxepv/trouble_installing_php_composer_and_laravel/ )

PHP Warning: PHP Startup: Unable to load dynamic library 'curl' (tried: C:\php\ext\curl (The specified module could not be found), C:\php\ext\php_curl.dll (The specified module could not be found)) in Unknown on line 0

I think it would be helpful if this message mentioned the extension_dir setting explicitly.

eg. something along the lines of "Is extension_dir correct?"

If you don't know that this setting exists, it's not obvious why PHP is looking for extension files in the wrong place.

I think it would also help to move the extension_dir setting directly above the extension= list (if possible) in the php.ini template files. (It is mentioned in the comments above, but those are easy to miss)

Start by locating the code that emits the dynamic-library startup warning and the php.ini template files mentioned in the issue. Check how extension_dir and extension entries are presented, then verify that the warning explicitly points users to extension_dir and the template places that setting directly above the extension list.