How to check if date is a monday (PHP)

how can I check if my date is a monday ?

Thats the date I want to check: 2020-04-20 00:00:00

I tried this, but it doesn't work:

if(date('D', $datum) === "Mon")
{
  echo "yes!";
}

Can someone please help me there, or show me an other way to check it ?