title: How to get the array keys of arrays inside an array
body: <p>I have a collection that has 2 arrays like on this picture:</p>
  <p><a href="https://i.stack.imgur.com/W5VWF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/W5VWF.png" alt="enter image description here" /></a></p>
  <p>So there are:</p>
  <pre><code>array 63 has two keys 70 and 72
  array 64 has key 71
  </code></pre>
  <p>What i want to do is to get the keys 70, 72, 71.</p>
  <p>How to get those keys??</p>