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