Hi, i had the same problem yesterday in my cplugin.php and i was about to get insane.
I fixed renaming the plugin folder, then i got into my admin panel, and when i review all my plugins, there was a plugin that i didn't installed before called :
Custom Code plugin
Cplugin Shows cusom codes with many options to display your ad codes.
Versión 1.0 | Por Rian Lambacher
Yesterday i just had downloaded Elementor, from this site, and i thought it was the problem, so i deleted the plugin too. I don't know how could it happened.
But at least it's solved