Incorrect Value Being Retrieved from the Database

Incorrect Value Being Retrieved from the Database

Situation: I created a cron job. It was to perform add orders entered by
customers to an API. I set it to run after every one minute.
Problem: Sometimes, when there were too many pending orders, second cron
job was being called before first one gets ended. It was resulting in
duplicate orders sent through API.
Solution: I created a table named as cron_jobs in my database. It has two
columns, ID and status. When a cron job runs, it checks whether the status
is 0 or not. If status is 0, it updates the status as 1. It completes its
process and marks the status again as 0. If the status of a job is 1 (it
means it is already running), and it is hit again, the operation is
terminated.
Unexpected Trouble: This solution seems logically correct. But when I
implemented it, the results were shocking. When a job is run, its status
is updated. But when it is invoked again, database still returns status 0
and it keeps on executing. I have also used sleep method so that I have
enough time to investigate the issue. But the database always returns
incorrect value than what I can see using phpmyadmin. Have a look at my
code:
<?php
$Output = mysql_fetch_assoc(mysql_query("select status from cron_jobs
where ID='1'"));
if($Output['status'] == '0')
{
mysql_fetch_assoc(mysql_query("update jobs set status='1' where ID =
'1'"));
mysql_fetch_assoc(mysql_query("update jobs set invoked = invoked+1
where ID = '1'"));
echo 'Executed';
}
else
{
echo 'Error: Another cron job already in process. Operation terminated!';
die();
}
/*I perform some lengthy tasks here*/
/*Sleep function called to check whether another instance of the
program works while this one is in progress */
sleep(60);
/*Task is complete. We are marking 0 as status so that another
instance is allowed to work on.*/
mysql_fetch_assoc(mysql_query("update jobs set status='0' where ID =
'1'"));
?>