Only replying as this comes up as the top result looking for digitalocean loadbalancer health check and others may find it useful.

As far as I'm aware TCP checks the port accepts a connection only, whereas HTTP looks for a 200 response I believe. I've only ever used the HTTP check.

We've used the following code as our health-check.php originally.

<?php
  $config = [
     "server"     => "172.x.x.x:3306",
     "username"   => "healthcheck",
     "password"   => "password"
  ];
  header('Content-type: application/json');

  $link = new mysqli($config['server'],$config['username'],$config['password']);
  $mount = is_readable("/mnt/www/index.php");

  if (($link->connect_errno) || (!$mount)) {
    http_response_code(503);
    echo('{ "status": "Unable to connect" }');
  }
  else {
    http_response_code(200);
    echo('{ "status": "Connection successful." }');
  }

?>

I'm not entirely sure if I found the above code online or had to build it up from different bits, if this is someone else's I'm happy to give credit.

It basically makes sure that the web server gets no errors when connecting to the database server and the filesystem mount is working. If either fails the loadbalancer takes that server out of action.
WARNING: If you put this on all servers and the DB or Filesystem goes down then ALL servers will be taken from the loadbalancer. You get a rather white page with something like 'unable to handle request' which isn't good for visitors.

To deal with the issues in the warning we replaced the above with:

<?php
function Test_Database() {

  $config = [
     "server"     => "172.x.x.x:3306",
     "username"   => "healthcheck",
     "password"   => "password"
  ];

  $link = new mysqli($config['server'],$config['username'],$config['password']);

  return $link->connect_errorno;
}

function Test_Mount() {

  $mount = is_readable("/mnt/www/index.php");

  return $mount;
}

function Test_UpTime() {
  $str   = @file_get_contents('/proc/uptime');
  $num   = floatval($str);

  return $num;
}

function Always_Online_Server() {

  $hostname = gethostname();

  if ( $hostname == "SERVER01" ) {
    return true;
  } else {
    return false;
  }
}

  $db = Test_Database();
  $mount = Test_Mount();
  $aos = Always_Online_Server();
  $uptime = Test_UpTime();

  header('Content-type: application/json');

  if ( $aos ) {
    http_response_code(200);
    echo('{ "status": "Connection successful." }');
  } else {
    if (($db) || (!$mount) || ($uptime < 300)) {
      http_response_code(503);
      echo('{ "status": "Unable to connect" }');
    }
    else {
      http_response_code(200);
      echo('{ "status": "Connection successful." }');
    }
  }
?>

I'll admit it's not the nicest code :) but this would leave 1 server as always reporting OK to the loadbalancer regardless of the database and mount. That way if wordpress has a database connection problem you can customise that error page. The server stills fail and is taken away from the loadbalancer if the web or php services are down. There's also the added check to make sure the servers have been up for more than 5 mins to allow new servers some time to settle before reporting OK to the loadbalancer.

I do hope this helps someone.