Adding leading zeroes to a string date in PHP

Adding leading zeroes to a string date in PHP

I have a string "date" which can be DD.MM.YYYY or D.M.YYYY (with or
without leading zeros), it depends what a user types. Then I use it in a
condition to send another email when the day is today.
if($_POST["date"]== date("d.m.Y")){
$headers.="Bcc: another@mail.cz\r\n";
}
The problem is that the mail is send when the date format DD.MM.YYYY (with
leading zeros) only.
My proposed solution
As I'm not very good in PHP, I only know the solution theoretically but
not how to write the code - I would spend a week trying to figure it out
on my own.
What's in my mind is dividing the date into three parts (day, month,
year), then checking the first two parts if there's just one digit and
adding leading zeros if it's the case. I don't know how to implement that
to the condition above, though. I have read a few topics about how to do
this, but they were a bit more different than my case is.