PHP DateInterval. Go back to the beginning of the year - php

Question

I would like to write a method which I can give a period of time (Like: yearly, monthly...) and it returns me an anterior date according to this period of time given.
Here is my code:
public function callRuleCeilling($period)
{
$start = new \DateTime();
switch ($period) {
case 'weekly':
$dateInterval = 'P7D';
break;
case 'monthly':
$dateInterval = 'P1M';
break;
case 'quaterly':
$dateInterval = 'P3M';
break;
case 'half-yearly':
$dateInterval = 'P6M';
break;
case 'yearly':
$dateInterval = 'P1Y';
break;
default:
$dateInterval = 'P1Y';
}
$start->sub(new \DateInterval($dateInterval));
return $start
}
My example problem:
If I put a starting date in the middle of the year with a yearly period. I want it to stop at the beginning of the year.
And I would like the same for monthly period (Stop at the beginning of the month) etc...
Does it exist a PHP function with do that? I can't find it.
Please highlight me.

Answer 1

Thanks fo the highlight. It allowed me to did that way:
public function callRuleCeilling($period)
{
$start = new \DateTime();
$month = 'January';
switch ($period) {
case 'weekly':
$timestampMonday = strtotime('last monday', strtotime('tomorrow'));
$start = $start->setTimestamp($timestampMonday);
break;
case 'monthly':
$month = $start->format('F');
$start = new \DateTime('first day of '.$month);
break;
case 'quaterly':
$monthNumber = $start->format('n');
if($monthNumber >= 1) $month = 'January';
if($monthNumber >= 5) $month = 'May';
if($monthNumber >= 9) $month = 'September';
$start = new \DateTime('first day of '.$month);
break;
case 'half-yearly':
$monthNumber = $start->format('n');
if($monthNumber >= 1) $month = 'January';
if($monthNumber >= 7) $month = 'July';
$start = new \DateTime('first day of '.$month);
break;
case 'yearly':
$start = new \DateTime('first day of January');
break;
default:
$start = new \DateTime('first day of January');
}
return $start;
}