title: Implode array value to string by key in laravel
body: <p>i have an array like</p>
  
  <p><img src="https://i.stack.imgur.com/cp2Ch.png" alt="Array Picture"></p>
  
  <p>how to implode array by key.</p>
  
  <pre><code>1001 =&gt; ['1,5,6...']
  1002 =&gt; ['2,3,4...']
  </code></pre>
  
  <p>How would I achieve this?</p>