You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '.ru' at line 1

Собственно сабж. Server version: 5.7.27-0ubuntu0.18.04.1 (Ubuntu)

define("NEWDB" , "site.ru");
$dbcnx = mysqli_connect($DBhost,$DBuser,$DBpass);
$createDB = "CREATE DATABASE ".NEWDB;
if (mysqli_query($dbcnx, $createDB)) {
    echo "Database ".NEWDB." created successfully<br>";
} else {
    echo "Error creating database: " . mysqli_error($dbcnx).'<br>';
}

Ну и БД не создается.