PHP array_push将一个数组转换成另一个数组

Question

I need to add one array inside of another using array_push or something similar.

What I would like to achieve will look like this:

$years = array(
    "2017" => array("Value01", "Value02"),
    "2016" => array("Value03")
);

What would be the best way to create an array within another? I have tried the following:

array_push($years, "2016" => array());

This just gives a

500 error.

And:

array_push($years, "2016");

This does add a new array within $yearshowever it is called "2018" and the "2016" value is place inside that 2018 array. I have manually created the 2017 array already, so I assume it is just incrementing on that number for some reason.