What does the portion of the code mean?

I was reading a code about PHP calendar and couldn't understand a small portion of it. Blow is the code.

<?php
/* Draws a calendar*/
function draw_calendar($month, $year) {
    /* Draw table*/
    $calendar = '<table cellpadding="0" cellspacing="0" class="calendar">';
   
    /*Table heading*/
    $headings = array("Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday");
    $calendar .= '<tr class="calendar-row"><td class="calendar-day-head">'.implode('</td><td class="calendar-day-head">',$headings).'</td></tr>';
    /*row for week one*/
    $calendar .= '<tr class="calendar-row">';
    /*Days and weeks vars now*/
    $running_day = date('w',mktime(0,0,0,$month,1,$year));
    $days_in_month = date('t',mktime(0,0,0,$month,1,$year));
    $days_in_this_week = 1;
    $day_counter = 0;
    $dates_array = array();
   
    /*Print blank days until first of the current week*/
    for ($x=0;$x<$running_day;$x++) {
        $calendar .= '<td class = "calendar-day-np"> </td>';
        $days_in_this_week++;
    }
   
    /*Keep going with days...*/
    for($list_day=1;$list_day<=$days_in_month;$list_day++) {
        $calendar .= '<td class = "calendar-day">';
        /*Add in the day number*/
        $calendar .= '<div class = "day-number">' . $list_day. '</div>';
       
        /*Query database for an entry on this day. If found, print them on them*/
        $calendar .= str_repeat('<p> </p>', 2);
       
        $calendar .= '</td>';
        if ($running_day == 6) {
            $calendar .= '</tr>';
            if (($day_counter+1) != $days_in_month) {
              $calendar .= '<tr class =" calendar-row">';
            }
            $running_day = -1;
            $days_in_this_week = 0;
        }
        $days_in_this_week++; $running_day++; $day_counter++;
    }
   
    /*Finish the rest of the days in the week*/
   
    if($days_in_this_week < 8) {
        for ($x=1;$x<=(8-$days_in_this_week); $x++){
            $calendar .='<td class ="calendar-day-np"> </td>';
        }
    }
   
    /*Final row*/
    $calendar .= '</tr>';
   
    /*End of table*/
    $calendar .= '</table>';
   
    /*all done, return result*/
    return $calendar;
}

Code (markup):

The first thing is that why the $dates_array variable has been created?
What is this portion of the code is doing?

if ($running_day == 6) {
            $calendar .= '</tr>';
            if (($day_counter+1) != $days_in_month) {
              $calendar .= '<tr class =" calendar-row">';
            }
            $running_day = -1;
            $days_in_this_week = 0;
        }
        $days_in_this_week++; $running_day++; $day_counter++;
    }

Code (markup):

The most important thing I want to understand is that at the end of the if statement, why the variables are being incremented to one.

Thank you.

 

This is the codebit that makes the table-rows stop and start over when it reaches the end of a week. The counters are a bit... weird, and such, but it's pretty straight forward. A count for running-day from -1 to 5 gives 7 (-1,0,1,2,3,4,5) and the day-counter counts the total of days passed in the current month, and checks if there are more days in the current month. The days_in_this_week-variable is being used to fill out the last row with empty <td>s after the last date is displayed.

Pretty straight forward calendar-stuff, but the numbers chosen are a bit weird, and could have been done easier.

EDIT: oh, yeah, and the $dates_array is declared, but not used - I dunno why, probably something that's put there for a "might be useful" or used by some other function not present in the current code