I have a field in database that has type of datetime in which I add time when user visit a page. When user again comes I want to check the interval between his first visit and c
What you can so is, retrieve the the datetime, store it in a variable. Create a var with time().
You can then convert the db datetime to a timestring using strtotime()
Subtract the datetime timestring from the new time. That should give you a difference in seconds. You can then manipulate your values and do the relevant checks.
$db = datetime_from_database;
$now = time();
$last = strtotime($db);
$diff = $now - $last; //this is in seconds
You can do something like
$minutes = $diff / 60;
If ($minutes > 60) echo 'more than 1 hour; 60 minutes';
Just work in it.
You can then use the date functions to format the new datetime using the $now and update the database.