title: Get unique keys found in the second level of a 2d array and sum the total number of occurrences
body: <p>Let's say I have an array like this:</p>
  <pre><code>$array = [
     ['a' =&gt; 'v', 'b' =&gt; 's', 'c' =&gt; 's'],
     ['c' =&gt; 's', 'd' =&gt; 's', 'b' =&gt; 's'],
     ['b' =&gt; 's', 'e' =&gt; 's', 'g' =&gt; 's'],
  ];
  </code></pre>
  <p>I want to create a new array which contains how many unique keys are found anywhere in my array's associative rows.</p>
  <p>Desired result:</p>
  <pre><code>[
      'a' =&gt; 1,
      'b' =&gt; 3,
      'c' =&gt; 2,
      'd' =&gt; 1,
      'e' =&gt; 1,
      'g' =&gt; 1
  ]
  </code></pre>
  <p>I tried with <code>array_count_values()</code>, but it is not right for my array structure.</p>