PHP How to extract first days of the weeks from a Month from Unix timestamp

There is this Unix timestamp and it needs to generate the first days of the week as an array.

    $time = '1456034400'; 
    // This present Month February 2016
    // in calendar the February has the start of the week 
    // Sunday 7
    // Sunday 14
    // Sunday 21
    // Sunday 28

How do you get an array like this from the Unix Timestamp:

    $weekdays = array(
                  0 => 7,
                  1 => 14,
                  2 => 21,
                  3 => 28
                 );

And this method needs to work and be accurate for any given month in years not just Feb 2016.