title: Collection reduce and sum by key
body: <p>I have a Collection like:</p>
  <pre><code>#items: array:2 [
      0 =&gt; Illuminate\Support\Collection {
        #items: array:1 [
          1 =&gt; 3
        ]
      }
      1 =&gt; Illuminate\Support\Collection {
        #items: array:2 [
          1 =&gt; 1
          3 =&gt; 2
        ]
      }
      2 =&gt; Illuminate\Support\Collection {
        #items: array:2 [
          1 =&gt; 2
          2 =&gt; 2
        ]
      }
    ]
  </code></pre>
  <p>and I somehow want to reduce this Collection only to one entry with <code>sum</code> by key.
  Result should look like:</p>
  <pre><code>#items: array:1 [
      0 =&gt; Illuminate\Support\Collection {
        #items: array:2 [
          1 =&gt; 5
          2 =&gt; 2
          3 =&gt; 2
        ]
      }
    ]
  </code></pre>
  <p>How can I achieve this with Laravel?</p>
  <p>Thanks!</p>