Suppose i know that today's day is monday. 
How do i use mktime() in php to get unix timestamp for last friday and the friday before that?? 
suppose today's date is 17-01-2011 and its a monday. Then i want the timestamp for 14-01-2011 00:00:00 and 7开发者_如何学运维-01-2011 00:00:00.
check out strtotime http://php.net/manual/en/function.strtotime.php ..solves most of that kinda issues - otherwise you must strap the date yourself
Something like this should do
<?php
    // Check if today is a Monday
    if (date('N') == 1)
    {
        // Create timestamp for today at 00:00:00
        $today = mktime('0', '0', '0', date('n'), date('j'), date('Y'));
        $last_friday = $today - 60*60*24*3;
        $last_last_friday = $today - 60*60*24*10;
        // Convert to a readable format as a check
        echo 'Last Friday\'s timestamp is ' . $last_friday . ' (' . strftime('%d-%m-%Y %H:%M:%S', $last_friday).') <br />';  
        echo 'Last last Friday\'s timestamp is ' . $last_last_friday . ' (' . strftime('%d-%m-%Y %H:%M:%S', $last_last_friday).')';  
    }
A lot more simpler than you thought (or even I for that matter)! Essentially, strtotime("last friday", time()) gets the timestamp of... last friday!
Getting the current timestamp first:
$unixNow = time();
echo date('r', $unixNow) . "<br />";
//Thu, 10 Jan 2013 15:14:19 +0000
Getting last friday:
$unixLastFriday = strtotime("last friday", $unixNow);
echo date('r', $unixLastFriday) . "<br />";
//Fri, 04 Jan 2013 00:00:00 +0000
Getting the friday before that:
$unixFridayBeforeThat = strtotime("last friday", $unixLastFriday);
echo date('r', $unixFridayBeforeThat) . "<br />";
//Fri, 28 Dec 2012 00:00:00 +0000
 
         
                                         
                                         
                                         
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