array_push() with key value pair

I have an existing array to which I want to add a value. I'm trying to achieve that using `array_push()` to no avail. Below is my code: ``` $data = array( "dog" => "cat" ); array_push($data['cat'], 'wagon'); ``` What I want to achieve is to add cat as a key to the `$data` array with wagon as value so as to access it as in the snippet below: ``` echo $data['cat']; // the expected output is: wagon ``` How can I achieve that?