Easy config file to use folder as subdomain, just put it in /etc/nginx/conf.d/

vhost.conf

server {
    listen 80;

    root /var/www/html/$subdomain;
    index index.php index.html index.htm;

    server_name ~^(?<subdomain>.+).knowlet.me$;

    location / {
        try_files $uri $uri/ =404;
    }

    error_page 404 /404.html;
    error_page 500 502 503 504 /50x.html;

    location = /50x.html {
        root /usr/share/nginx/html;
    }

    location ~ \.php$ {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
        fastcgi_index index.php;
        include fastcgi_params;
    }

    location ~ /\.ht {
        deny all;
    }
}