PHP - Does This Mean 30 Mins?

I have a database with Users as a table. It has the normal stuff, id, username, password, and I have anther in it called lastactive. It is DATETIME, and I am wondering how I would make a sql query select the users in the database 'users' and see who has been active in the past 15 mins.

What I am asking is how I could do this, and would it be better to use a different structure other then DATETIME.

Im still trying to work this one out but what i would like to do is send an email if an alert is present. but not resend the email on every page refresh where the alert is still present.

so i was thinking of using something like $_SESSION['sent_time'] to record when the last email was sent and only send another email if the current time is greater than 10 mins from the $_SESSION['sent_time'].

How can i record the time into the session variable in a format which i can use to compare with the current time?
and how will i compare?

cheers for any suggestions

Hi guys,

I will like to insert a countdown timer of 5 mins after form has been submitted to a php page.

At the moment when they submit the form they see the date and time they submitted:

echo date("M d Y  H:i", time()) ";

Any ideas?