Is this wordpress virus?
Never trust unfamiliar code that you didn't include! Scammers often try to make malware look innocuous, like above. Try installing WordFence, Sucuri, or another security plugin and run a scan.
That looks like your server has been hacked I had to deal with similar issues one or two times in the past. And yes the hack seems like it is specifically targeted to WordPress sites.
The WordPress core should never be modified anyway.
Change all your server passwords (customer center, FTP, SSH, SQL, whatever)
Download a fresh installation of the WordPress core and remove the old one
Check your wp-content folder for security issues especially your Theme and your Plugins (check for any modified or insecure code)
Change your security salts in wp-config.php
After a fresh setup of your WordPress site increase its security. There are various articles on how to secure WordPress and there are several WordPress security Plugins available. Scan your site.
If a similar problem appears again you should also check the security settings of your webserver / webhost.