title: flat a laravel collection by a specific index
body: <p>Suppose I have a a collection like this : </p>
  
  <pre><code>Collection {#447
    #items: array:3 [
      0 =&gt; array:6 [
        "pos" =&gt; "0"
        "col" =&gt; "1"
        "row" =&gt; "1"
        "size_x" =&gt; "2"
        "size_y" =&gt; "1"
        "cat_id" =&gt; "1"
      ]
      1 =&gt; array:6 [
        "pos" =&gt; "0"
        "col" =&gt; "3"
        "row" =&gt; "1"
        "size_x" =&gt; "1"
        "size_y" =&gt; "1"
        "cat_id" =&gt; "11"
      ]
      2 =&gt; array:6 [
        "pos" =&gt; "0"
        "col" =&gt; "1"
        "row" =&gt; "2"
        "size_x" =&gt; "2"
        "size_y" =&gt; "1"
        "cat_id" =&gt; "10"
      ]
  ]
  }
  </code></pre>
  
  <p>Now I want an array like this that get only <code>cat_id</code> indexes. </p>
  
  <pre><code>[1,11,10]
  </code></pre>
  
  <p>How can I do that in laravel ? </p>