title: Merge Laravel collections keys in only string
body: <p>I have this Laravel 5.3 tinny collection:</p>
  
  <pre><code>Collection {#325 ▼
    #items: array:8 [▼
      0 =&gt; 2638
      1 =&gt; 2100
      2 =&gt; 5407
      3 =&gt; 2970
      4 =&gt; 4481
      5 =&gt; 1611
      6 =&gt; 5345
      7 =&gt; 50
    ]
  }
  </code></pre>
  
  <p>And I want combined in only string the values, I need this:</p>
  
  <pre><code>"2638,2100,5407,2970,4481,1611,5345,50"
  </code></pre>