Мне кажется, я понимаю, что вы хотите.
$ref = strtotime('2011-11-28');
$today = strtotime('2011-12-19');
/* Your selected Monday. */
$week = 3600 * 24 * 7; /* Number of seconds in a week. */
$number = ($today-$ref)/$week;
/* You could ceil() or floor() this, depending on how you want it to behave. */