allow only single process to execute my script instead of having more than one process executing the same script

Hi Experts,

I wrote a bash script that will execute my php code every 30 min via cron job. For some reason when check the process via the ps aux | grep myphp.php, I can see more than one process that are running the same php code. Is there a way for me to ensure that there is only a single process that executes my code instead of having more than one process running the same code? Thanks