Adding Values to an Array

How can I add a key/value set to an array?

I tried using array_push, but it doesn't seem to be working. Here is what I tried:

$string = "300=>null";
$data = array( '200'=>null );
array_push($data, $string );
print_r($data);

My results are:

Array ( [200] => [201] => '300'=>null )

I'd like my results to be:

Array ( [200] => [300] => )

Any assistance is greatly appreciated.