I got annoyed finding something simple to find the date for monday in the current week (easily adapted for to find the monday for any given date) - so I gave up being lazy, and whipped up my own quick and dirty example. Please be gentle - I'm a noob obviously lol.
Putting it here also keeps it stored for my future use too
// Get the numerical representation for the current weekday where 1=monday, 7=sunday.
// We subtract 1 to avoid an over-subtraction in the next line.
// Gets the timestamp for monday by counting how many days we are currently
// away from monday (less 1, as above)
// Just for demo purposes, the formatted date
echo date('d M Y',$mon);