Use of undefined constant REQUEST_URI - assumed 'REQUEST_URI' in functions.php on line 73

$path = $_SERVER[‘HTTP_HOST’] . $_SERVER[REQUEST_URI];

to

$path = $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];

Warning: Use of undefined constant REQUEST_URI - assumed 'REQUEST_URI' in /www/docs/wordpress/wp-content/themes/twentyfifteen/functions.php on line 73

The above error show on my site https://cartvela.org/ and I am changing

$path = $_SERVER[‘HTTP_HOST’] . $_SERVER[REQUEST_URI];

to

$path = $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];

With that code adjustment, my site comes back and error is gone

Be very careful when this warning occures because this can be a side effect of the wp-vcd.php malware. Check if the file wp-includes\wp-vcd.php exists. If yes, you are infected and you should not fix this warning but instead reinstall wordpress. And avoid installing nulled themes/plugins.