finding Date difference in php

[rosy]

Hi,
How to find difference between two dates.
I have to find if install date is less than 8 weeks from the date of entry.
How can i find this, my code is below.

<? php
if($_POST[submit]){
$install_date_entered = $_POST[aninstallyear]."-".$_POST[aninstallmonth]."-".$_POST[aninstallday];
$new_date_entered = $_POST[astartyear]."-".$_POST[astartmonth]."-01";

$datediff = ($install_date_entered - $new_date_entered ) ;

echo " The Difference between dates  = $datediff";
}
?> 

The date is selected using combo box.Year is selected seperately, month is selected seperately and day is selected seperately.
please help me in finding the difference between dates
Thanks

[NogDog]

function week_diff($date1, $date2)
{
   $time1 = strtotime($date1);
   if(!$time1)
   {
      user_error("week_diff(): invalid date/time for 1st arg", E_USER_WARNING);
      return(FALSE);
   }
   $time2 = strtotime($date2);
   if(!$time2)
   {
      user_error("week_diff(): invalid date/time for 2nd arg", E_USER_WARNING);
      return(FALSE);
   }
   return((strtotime($date1) - strtotime($date2)) / (7*24*60*60));
}
$numWeeks = week_diff($install_date_entered, $new_date_entered);