Firstly thanks for your kind words! :)

So essentially, it sounds like you've been using the port binding as something of a process lock?! Assuming I'm right, whilst I agree that it's quite hacky, it's also quite inventive! :) It's not something that I would have considered.

I'm unfamiliar with anything but the most rudimentary PHP so I can't really comment on the "best" way of acheiving your ends. But I can think of a couple of pathways which would be less hacky.

option 1: triggered by a cron job, leveraging a PID file

First up, if I were creating a script to do what you're doing, I'd be inclined to generate a "PID" file (a file which stores your script's "Process ID" - every running process get's assigned a PID). Ideally you'd want to trap errors too, so that even if the script fails, it would clean up the PID file on exit (so the file definitely only exists when the process is running; or the edge case of a complete server crash).

Due to my complete lack of familiarity with PHP and the fact that I haven't explicitly done any of that sort of stuff for a while, I couldn't really offer any concrete suggestions on how to achieve that. But google might help (now you know keywords such as "pid file" & "trapping errors", etc). Armed with that, I'd be surprised if you couldn't find plenty of good info, plus keep in mind that TurnKey is based on Debian (v14.x = Debian Jessie, upcoming v15.x will be based on Stretch).

Personally, I'd be inclined to write a bash wrapper script for your PHP script and let that manage the error trapping and PID file creation/deletion etc. But OTOH that may not necessarily be the best way, it's just what I'd do. And in fairness, that's because I'm very familiar with bash! :)

Regardless, once you have a "proper" pid file plus error trapping sorted out, then a cron job is certainly a good option for this task (a cron job is essentially like a Windows "task" - triggering a task on a timed schedule). That's where the importance of a PID file & error trapping comes in. So each time the cron job is triggered, it can ensure your script isn't already running. Otherwise, you'll encounter the same issues you had before, with multiple instances of your script running.

I'd also recommend some form of logging, so if it does fail, you'll have a relatively easy way of working out what went wrong. A little extra initial development work, will likely save you a lot of headaches down the track. If you do decide to do logging, I highly recommend that you look into leveraging 'logrotate'. That can be configured to compress and/or delete old logs to avoid them growing uncontrollably. Again google will be your friend! :)

option 2: a systemd service file

Assuming that you are using TurnKey v14.x (anywhere other than an Proxmox/LXC container; earlier versions of TurnKey & v14.x LXC used an older init system called sysv-init) then another pretty incredible option IMO would be to turn this into a SystemD service (aka daemon).

You could essentially keep your looping script as is (perhaps include a sleep timer if you don't need/want it running all the time) and write a SystemD service file to trigger it. You could then enable it to auto start on boot ('systemctl enable php-script') and could easily check the state of it anytime you liked ('systemctl status php-script'), as well as manually stop ('systemctl stop php-script') and start it ('systemctl start php-script'). You could even make it auto stop under certain conditions, e.g. if MySQL is stopped). Whilst the learning curve may be a little steeper initially, you could then avoid needing to worry about the PID file and would get (albeit rudimentary) logging for free! SystemD is pretty damn awesome IMO!

A PID file is not necessary, as SystemD will keep track of your process. Note that if you still use a bash wrapper, then ensure that you launch your PHP script with the 'exec' command! Otherwise SystemD will only watch the bash script, not the PHP one too. If you can be bothered, I'd still be inclined to set up some logging (even if it's disabled by default). That will make debugging any future issues significantly easier!

getting further help

No matter which way you go, I highly recommend Server Fault (server related questions), 'nix Stack Exchange (Linux specific questions) and Stack Overflow (programming related questions) (and Stack Exchange sites in general) for any specific questions which google doesn't answer for you. As there is a bit of overlap between these sites, I suggest that you consult the help (append /help on the end of each URL) to avoid the risk of your questions being closed as off topic. I also suggested trying to keep your questions as tightly targeted as possible. I suggest writing a few separate short questions, rather than try to get everything answered in one.

Please don't take that last paragraph to be me trying to discourage you from asking here though. I'm more than happy to help as much as I can (I quite enjoy it). It's just you'll likely get a quicker (and perhaps even better) answer to your more generic questions there, than you will here. My knowledge tends to be quite broad, but not very deep (beyond TurnKey specifics). Plus you won't have to wade through my rambling posts...! :)

So please feel free to post back if you get stuck. Also it'd be awesome if you share your solution once you have something you're happy with. I have no doubt that other TurnKey users would get some value out of it!

Good luck!