PHP - Lastactive Users Up To 15 Minutes

Hello   I am trying to work out how many regular users I have to my site and how long those users tend to be users..   So, I have a table that logs every time a user visits my site and logs in, it stores the date / time as a unix timestamp and it logs their user id.   I started by getting the id's of any user who logs in more than 5 times in a specified period, but now I want to extend that...  

 
SELECT userID as user, count(userID) as logins FROM login_history where timestamp > UNIX_TIMESTAMP('2014-06-01 00:00:00') and timestamp < UNIX_TIMESTAMP('2014-07-01 00:00:00') group by user having logins > 5;
 

I just discovered that I have a major security flaw with my website.
Anyone who logs in to the website can easily access other users information as well as delete and edit other users information just by changing the ID variable in the address bar.
I have user ID Session started on these pages but still people can do anything they like with other users information just by editing the address bar.
For example if your logged in in the address bar of www.mywebsite.com/delete_mystuff.php?id=5 and change the "5" say to a "9" then you will have access to user#9 information.

Every important page that I have has this code:
Code: [Select]
 session_start();

if (!isset($_SESSION['user_id'])) {
// Start defining the URL.
         $url = 'http://' . $_SERVER['HTTP_HOST'] . dirname($_SERVER['PHP_SELF']);
// Check for a trailing slash.
         if ((substr($url, -1) == '/') OR (substr($url, -1) == '\\') ) {
         $url = substr ($url, 0, -1); // Chop off the slash.
}
// Add the page.
$url .= '/index.php';
ob_end_clean(); // Delete the buffer.
header("Location: $url");
exit(); // Quit the script.
} else {
                //Else If Logged In Run The Script

if((isset($_GET['id'])) && (is_numeric($_GET['id']))) {
         $id = (int) $_GET['id'];
} elseif ((isset($_POST['id'])) && (is_numeric($_POST['id']))) {
         $id = (int) $_POST['id'];
} else {
         echo ' No valid ID found, passed in url or form element';
        
         exit();
}



What am I doing wrong?
Please help if you know how to correct this.

Many thanks in advance.

To get current date and time I use:

$date = date("d-M-Y h:i:s");

But how can I get date and time which was before 5 minutes.

I'm trying to add some minutes ($interval) to any given time, but my results aren't as expected...

Code: [Select]
<?php
$year="2012";
$opentime="09";
$interval="10";
$thedate = "$year:01:01 $opentime:00:00";
$startdate = strtotime($thedate);

$date = date("Y-m-d",$startdate);
$time = date("g:i a",$startdate);

$newtime = strtotime("+$interval", $time);
$nexttime = date("g:i a",$newtime);

echo $date;
echo "<br>";
echo $time;
echo "<br>";
echo $nexttime;
is returning:
2012-01-01
9:00 am
7:10 pm

I was expecting the last to be 9:10 am.
Can someone point me in the right direction? Thanks.

hello, if i have the following, how would i display it?

Code: [Select]
function minutes_round ($hour = "$signintime", $minutes = '5', $format = "H:i")
{
    $seconds = strtotime($hour);
    $rounded = round($seconds / ($minutes * 60)) * ($minutes * 60);
    return date($format, $rounded);
}