Unexpected notice: undefined index when null - symfony

Question

I've written a little script that is supposed to create an array that looks something like this:
array(1 => array( 'month' => 'Jan',
'salary' => '01/31/2013',
'bonus' => '02/15/2013'),
2 => array('month' => '',...));
You get the basic idea: the index in the main array are the months (numerically), each holding an array that is to be filled dynamically. The month key depends on the language the user requested, the salary and bonus are assigned the dates when the salary and or bonus was payed. So far no surprizes.
To get the basic structure of that array, I thought this would be the easiest:
$this->months = array_fill_keys(range(1,12), array( 'month' => null,
'salary' => null,
'bonus' => null));
I then fill the array, and things run smoothly all the same, right up until the point where I want to write the data to a file, which I did like so:
private function writeFile()
{
foreach($this->months as $key => $vals)
{
if ($vals['month'] === null)
{//Only filled from date x to date y, some months can be empty
continue;
}
//this seems to raise notices?
if ($vals['salary'] === null)
{
$vals['salary'] = 'Does not apply';
}
fwrite($this->file, implode(',', $vals).PHP_EOL);
}
fclose($this->file);
return $this;
}
The line where I check if the salary is null raises notices: "Warning: Undefined index salary". At the moment I've had to add this to the code:
if (!array_key_exists('salary', $vals) || $vals['salary'] === null)
{
if (!array_key_exists('bonus', $vals) || $vals['bonus'] === null)
{
break;
}
$vals['salary'] = 'Does not apply';
}
To get to the result I need. I have googled this, and stumbled across this bug report, which was last modified 4 years ago (2009-05-08), but the status is still set to "no feedback".
Has anyone else encountered a similar glitch/bug? Or am I missing something here? How can I avoid this issue without the need for all too many if's and function calls without changing my settings (E_STRICT | E_ALL, as it should be).
BTW: I'm running PHP 5.4.7, on Slackware 14. For this little application, I'm using 2 Symfony components (ClassLoader and Console), but since this is part of an object that has nothing to do with Symfony, apart from being loaded through UniversalClassLoader I don't think that's relevant.
Since the bug is said to be PDO related: yes, I am using PDO, but in another class.

Answer 1

I am not sure , but try to use
$this->months = array_fill(1,12, array( 'month' => null,
'salary' => null,
'bonus' => null));

Answer 2

After a couple of var_dumps, I found out what the cause was: The array keys were range(1,12), to determine the which month was I was processing. For that I used a DateTime object in the following fashion:
$date->modify('+ '.$diff.' days');
$key = $date->format('m');
The problem was that the format call returns a string. The goal was to list when the salary and bonus are to be paid. The bonus has to be paid each 15th, or the following Wednesday, if the 15th was a Saturday or Sunday. The salary is to be paid the last day of the month, or the last Friday.
In other words, the bonus pay date is assigned like so:
$key = $date->format('m');
$this->months[$key]['month'] = $date->format('M');
if ($date->format('d') == 15)
{
//find find week-day (15th or following Wednesday)
$this->months[--$key]['bonus'] = $date->format('m/d/Y');
$key++;
//set date to end of month
}
//check week-day, and modify date if required
$this->months[$key]['salary'] = $date->format('m/d/Y');
Because the keys of the $this->months array are numeric, but the format used for $key is a 2-digit string, with leading zeroes, I was having problems.
Each 15th of the month, the $key value is coerced to an integer (decrement/increment operators), but the month is assigned using a string.
The information I provided in my original question was insufficient, sorry for that, but I had just put in an all-nighter. The fix, in the end was pretty easy:
$key = (int) $date->format('m');//cast
I sincerely appreciate all responses, and everyone contributing to the SO community. I would've deleted this question, but if nobody objects, I think I might leave it be as a testimony to my stupidity.