Array values replaced rather than pushed - php

Question

I'm trying to build a multidimensional array like array(years => array(months)). Each new month added to a year replaces the old month value in the year array rather than appending the value.
<?php
$dates = array();
foreach (Blog::all() as $blog) {
$date_year = date("Y", strtotime($blog->created_at));
$date_month = date("F", strtotime($blog->created_at));
if (!in_array($date_year, $dates)) {
$dates[$date_year] = array();
}
if (!in_array($date_month, $dates[$date_year])) {
$dates[$date_year][] = $date_month;
print $date_year." ".$date_month."<br>";
}
}
print_r($dates); ?>
Outputs:
Array ( [2009] => Array ( [0] => December ) [2010] => Array ( [0] => March ) [2011] => Array ( [0] => August ) [2012] => Array ( [0] => November ) [2013] => Array ( [0] => October ) [2014] => Array ( [0] => April ) )
The months displayed are the last month available for each year. I have also tried with array_push($dates[$date_year], $date_month) for the same result.

Answer 1

The year array is being overwritten each time here:
if (!in_array($date_year, $dates)) {
$dates[$date_year] = array();
}
Check the array keys instead of the array values with array_key_exists:
if (!array_key_exists($date_year, $dates)) {
$dates[$date_year] = array();
}
Or, better yet IMO, use isset as suggested by vp_arth below. (For succintness.)
if (!isset($dates[$date_year])) {
$dates[$date_year] = array();
}