<?php
function last_monday($date) {
    if (!is_numeric($date))
        $date = strtotime($date);
    if (date('w', $date) == 1)
        return $date;
    elseif (date('w', $date) == 0)
        return strtotime(
            'next monday',
             $date
        );
    else
        return strtotime(
            'last monday',
             $date
        );
}

echo date('m/d/y', last_monday('8/14/2012'));

?>

08/13/12