How to find the date of a day of the week from a date using PHP?

Try

$date = '2012-10-11';
$day  = 1;
$days = array('Sunday', 'Monday', 'Tuesday', 'Wednesday','Thursday','Friday', 'Saturday');
echo date('Y-m-d', strtotime($days[$day], strtotime($date)));

You can use the date() function:

date('w'); // day of week

or

date('l'); // dayname

Example function to get the day nr.:

function getWeekday($date) {
    return date('w', strtotime($date));
}

echo getWeekday('2012-10-11'); // returns 4

I think this is what you want.

$dayofweek = date('w', strtotime($date));
$result    = date('Y-m-d', strtotime(($day - $dayofweek).' day', strtotime($date)));