edit-pages.php and page-new.php downloading instead of executing

I’m having a problem that whenever I click Edit or Add New in the Pages section of the Dashboard nav, the PHP files download instead of executing. If I click Edit, edit-pages.php is downloaded; if I click New, page-new.php is downloaded.

This problem happens in multiple browsers, when all plug-ins are deactivated, and regardless of the theme I’m using. It doesn’t happen with any other WordPress scripts. I’ve replaced those files with freshly downloaded versions from the install zip. And I’ve run other test scripts in the wp-admin directory to make sure that it isn’t a permissions problem or config issue.

Help, please? I’m completely stumped.