WordPress Editor not updating files: Unable to communicate back with site to check for fatal errors

What worked for me is disable all plugins and then re-enable them. This was caused by a wordpress update. Everything is working as expected now, but it was quite frustrating..

PS: Use the bulk deactivate action.

This error comes from a Wordpress feature included in Wordpress 4.9. It basicaly check for PHP errors upon file uploading in the WP file editor to prevent saving code that could/will break your site.

It's mostly a plugin (or a server configuration) that prevent the Wordpress loopback check to properly check for potential errors caused by your update. As Wordpress is not able to check whether your new code will produce errors or not, it prevents the code update.

Check those links:

• https://wpengine.com/support/php-editor-errors-wordpress-4-9-higher/
• https://wordpress.org/support/topic/cant-edit-main-theme-php-files-after-upgrading-to-4-9/

You could try disabling your plugins to see if the feature make it back, or update your files using FTP/SFTP (without the Wordpress file editor).

I'm a little late to the party, and Mtxz is right on their answer. However, their solution doesn't "fix" the issue. I found this forum which says you can solve it by removing lines 492 - 599 in the wp-admin/includes/file.php file.

The first line is if ( $is_active && 'php' === $extension ) { and the third last line is return new WP_Error( 'php_error', $message, $result );, followed by two lines of brackets.

This fixed the issue on several of my servers.

Note: this solution also removes PHP debugging, so if you have a syntax error and you do this, your site might crash and you can't edit the files unless you go through SFTP/FTP. As always, it's best to have a backup before editing files.