Send Cron Job Notification By Timezone Php

I want to write a script that will send push notification to users' phone every day at 2 pm. But there are users from all over the world, so I need to send that notification at time depending upon the timezone of particular user, which I keep in DB like this 'GMT+4'.

I need to do this by Cron job. I have never used that, so I don't know how to write that script and make this work in PHP. Anybody can help me?

A very simple approach is to have a cron-job call a PHP script every 1 minute, and have it check the database for things that need to be done. If something needs done now(), then do it.

In crontab you can use something like this,

* * * * *   /usr/bin/php /path/to/my/script.php

And script.php can be like this

<?php
$ts = time();

// Run for up to 50 seconds
while($ts + 50 > time())
{
   ... SELECT id, time FROM Table WHERE time <= timeFromGMT(GMTvalue) ...
   if(returns a row){
     Send notification;
     Set a flag that notification is send;
   }
   sleep(5);       
} 

Note: Use some locking mechanisms to prevent multiple processes at the same time.