function GetDateDifference ($Date1,$Date2)
{
$ConvertToTimeStamp _Date1 = strtotime($Date1);
$ConvertToTimeStamp_Date2 = strtotime($Date2);
$DateDifference = (int)$ConvertToTimeStamp _Date1 – (int)$ConvertToTimeStamp_Date2;
return round($DateDifference/86400);
}

Calculate the difference between two Dates (and time) using Php. The following page provides a range of different methods (7 in total) for performing date / time calculations using Php, to determine the difference in time (hours, munites), days, months or years between two dates.

See Php Date Time – 7 Methods to Calculate the Difference between 2 dates

Calculate How Many Days User Has Stayed With The Book Once She Return It).
I am learning PHP/MySQL, and I am also writting the Library Management
System to make sure that I practice what I am learning.

My Problem is:

I want to capture the date when the library user borrow the book and then
check that date to see if the book have exceeded the number of days (to
calculate how many days user has stayed with the book once she return it).
Please help me how I can do that. I am using mysql as my database.