title: Count unique values in a column of an array
body: <p>I have an array like this:</p>
  <pre><code>$arr = [
      1 =&gt; ['A' =&gt; '1', 'C' =&gt; 'TEMU3076746'],
      2 =&gt; ['A' =&gt; '2', 'C' =&gt; 'FCIU5412720'],
      3 =&gt; ['A' =&gt; '3', 'C' =&gt; 'TEMU3076746'],
      4 =&gt; ['A' =&gt; '4', 'C' =&gt; 'TEMU3076746'],
      5 =&gt; ['A' =&gt; '5', 'C' =&gt; 'FCIU5412720']
  ];
  </code></pre>
  <p>My goal is to count the distinct values in the <code>C</code> column of the 2-dimensional array.</p>
  <p>The total rows in the array is found like this: <code>count($arr)</code> (which is <code>5</code>).</p>
  <p>How can I count the number of rows which contain a unique value in the 'C' column?</p>
  <p>If I removed the duplicate values in the <code>C</code> column, there would only be: <code>TEMU3076746</code> and <code>FCIU5412720</code></p>
  <p>My desired output is therefore <code>2</code>.</p>