$startDate = '1999-03-12';
 
$endDate = '2011-03-12';
 
 
$days = (strtotime($endDate)-strtotime($startDate)) / (60 * 60 * 24);
 
echo $days; //Will output 4383

In the above example we have a startdate and a enddate. We need to find out how many days are between the startdate and the enddate.

We convert both dates to a UNIX Timestamp using the strtotime PHP function.

After the conversion we substract the startdate from the endate and then divide it by the number of seconds in a day (60*60*24).

We then output the day difference between 2 dates by using the echo PHP command.