The string "2016" is interpreted as a two-digit hour and
two-digit minutes time, rather than a four-digit year.

To demonstrate this:

    echo date('H:i', strtotime('2016')); // 20:16

This case is explicitly mentioned in the "Date Formats" manual page [1]:

> Note: The "Year (and just the year)" format only works if a time 
> string has already been found -- otherwise this format is 
> recognised as HH MM.

[1] https://www.php.net/manual/en/datetime.formats.date.php

This definitely does not make sense from any perspective. 

I have not seen anyone use hour:minute time format without : as in 20:19. But literally everyone uses 2019 as represenation of years.

There is no other use case for 4 numbers but to be a year.

This should be revisited.

Now to get the correct year I have to do somersaults like:

if(strlen($year)==4){
  $year = $year.'-01-01';
}

echo date('Y-m-d', strtotime($year));

I don't see how this benefits anyone