Let me know in a comment if there is any incorrect data.

http://www.rdlt.com/postcodes-van-alle-gemeentes-van-belgie-met-gps-coordinaten

Enjoy!

/**
* Get Mondays and Sundays
*
* Get monday, sunday, last monday & last sunday
* Example usage:
* // to retreive the dates using today as starting point
* $mondaysAndSundays = getMondaysAndSundays();
* // to retreive the dates using a custom date as starting point
* $mondaysAndSundays = getMondaysAndSundays('1987-04-14');
*
* @param date $offset Provide a date from where to calculate from in strtotime() translatable format. If none is given, today's date will be used.
*
* @return array
*
*/
function getMondaysAndSundays($offset=false)
{

if(!$offset) $offset = strtotime(date('Y-m-d'));
else $offset = strtotime($offset);

// this week
if(date('w',$offset) == 1)
{
$mas['monday'] = date('Y-m-d',$offset);
}
else
{
$mas['monday'] = date('Y-m-d',strtotime("last Monday",$offset));
}

if(date('w',$offset) == 6)
{
$mas['sunday'] = date('Y-m-d',$offset);
}
else
{
$mas['sunday'] = date('Y-m-d',strtotime("next Sunday",$offset));
}

// last week
if(date('w',$offset) == 1)
{
$mas['lastmonday'] =  date('Y-m-d',strtotime('-1 week',$offset));
}
else
{
$mas['lastmonday'] = date('Y-m-d',strtotime('-1 week', strtotime(date('Y-m-d',strtotime("last Monday",$offset)))));
}

if(date('w') == 6)
{
$mas['lastsunday'] = date('Y-m-d',strtotime('-1 week',$offset));
}
else
{
$mas['lastsunday'] = date('Y-m-d',strtotime("last Sunday",$offset));
}

return $mas;
}