Confusing Function Output

I am really confused with the outcome of the following function. I know that its a small snippet of code but I think there is enough information to understand what is supposed to happen:

while ($k < $dateCount) {
        echo $k." ==> ".$forecastData[$k]['Booked Date']." ==> ".$startDate."<br>";
        if($forecastData[$k]['Booked Date'] == $startDate){
            echo $k." ==> YAY<br>";
        }
        else{
            echo $k." ==> boo ==> ".$startDate."<br>";
        }
        $k++;
        $startDate = date('Y-m-d', strtotime("+1 day", strtotime($startDate)));
    }

$k is the array key -> starts at 0;

$dateCount is the size of the array (to stop it at the 31 days or however long the range is)

$forecastData is a multidimensional array of all sales data for a date. – contains loads of information for that date including the booked date which is the day the information is stored for.

$startDate is the date that has been put in but converted to the same day last year – in this case the date put in is Oct 01 2016 and the converted date is Sep 26 2015

I want to say that if the dates are equal then echo the number of the array with the word YAY otherwise echo the array number with the word boo.

The k increases by 1 per loop as does the date.

I have also printed the initial values of the loop to show me what the comparison is.

What I cant understand is that this is the output:

0 ==> 2015-09-26 ==> 2015-09-26

0 ==> boo ==> 2015-09-26

1 ==> 2015-09-27 ==> 2015-09-27

1 ==> YAY

2 ==> 2015-09-28 ==> 2015-09-28

2 ==> YAY

3 ==> 2015-09-29 ==> 2015-09-29

3 ==> YAY

4 ==> 2015-09-30 ==> 2015-09-30

4 ==> YAY

5 ==> 2015-10-01 ==> 2015-10-01

5 ==> YAY

6 ==> 2015-10-02 ==> 2015-10-02

6 ==> YAY

7 ==> 2015-10-03 ==> 2015-10-03

7 ==> YAY

8 ==> 2015-10-04 ==> 2015-10-04

8 ==> YAY

9 ==> 2015-10-05 ==> 2015-10-05

9 ==> YAY

10 ==> 2015-10-06 ==> 2015-10-06

10 ==> YAY

11 ==> 2015-10-07 ==> 2015-10-07

11 ==> YAY

The first one makes NO sense to me?? It does equal what it should but for some reason skips by it?? Also if I hardcode the date in then it will work.

I have been trying to understand this for so long now that any ideas will help

Thank you