Hrm- it makes no sense to me. My bet would be file corruption.
Can you grab a fresh download and then delete and replace the ext.rte.php file?
If that doesn’t fix it, go back and open up the file and copy out line 19 and paste it here.
var $docs_url = DOC_URL.'add-ons/rte/control_panel/index.html';
That’s the line- and there’s no way that constant shouldn’t be defined- and it would throw a different error. Plus the same code is used all over the place.
In short- it’s odd.