core-bundle/src/Resources/contao/library/Contao/Date.php
public function getWeekBegin($intStartDay=0)
	{
		$intOffset = date('w', $this->strDate) - $intStartDay;

		if ($intOffset < 0)
		{
			$intOffset += 7;
		}

		return strtotime('-' . $intOffset . ' days', $this->strDate);
	}
public function getWeekBegin($intStartDay=0)
	{
		$intOffset = date('w', $this->strDate) - $intStartDay;

		if ($intOffset < 0)
		{
			$intOffset += 7;
		}

		return strtotime('-' . $intOffset . ' days', $this->strDate);
	}
Return the begin of the week as timestamp

@param integer $intStartDay The week start day

@return integer The Unix timestamp