script.php

<?php

echo "Start" . PHP_EOL;
sleep(60);
echo "Stop" . PHP_EOL;

script.sh

#!/usr/bin/env bash

_cleanup() {
  echo "Caught SIGTERM signal! Sending to child and exiting loop"
  kill -s INT "$child"
  wait
  exit
}
trap _cleanup SIGINT SIGTERM

while true; do
  php script.php &
  child=$!
  wait "$child"
done

echo "Done, exiting"