Hooks: Standardize naming of dynamic hooks to use interpolation vs concatenation.

Benefits gained in discoverability and self-documentation throughout core trump the negligible performance hit in using interpolation in hook names.

Props ramiy.
See #37748.


git-svn-id: https://develop.svn.wordpress.org/trunk@38307 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-login.php

@@ -426,7 +426,7 @@ do_action( 'login_init' );
  *
  * @since 2.8.0
  */
-do_action( 'login_form_' . $action );
+do_action( "login_form_{$action}" );
 
 $http_post = ('POST' == $_SERVER['REQUEST_METHOD']);
 $interim_login = isset($_REQUEST['interim-login']);