title: (Laravel 5.3) How to add new key and value to existing sub array in existing Collection?
body: <p>I am using Laravel 5.3.</p>
  
  <p>existing Collection $a is</p>
  
  <pre><code>$a = collect(
  [
   0 =&gt;[
        'firstName' =&gt; 'John',
        'lastName' =&gt; 'Doe'
       ],
  1 =&gt;[
       'firstName' =&gt; 'Mary',
       'lastName' =&gt; 'Jane'
      ]
  ]);
  </code></pre>
  
  <p>and desired result is as below:</p>
  
  <pre><code>$a = collect(
  [
   0 =&gt; [
         'firstName' =&gt; 'John',
         'lastName' =&gt; 'Doe',
         'occupation' =&gt; 'engineer'
        ],
   1 =&gt; [
         'firstName' =&gt; 'Mary',
         'lastName' =&gt; 'Jane',
         'occupation' =&gt; 'accountant'
        ]
  ]);
  </code></pre>
  
  <p>I tried to test ->push(), ->put(), ->prepend() but no success. Please let me know the best way to do this.</p>