title: Difference between string arrays in PHP
body: <p>Given two arrays of strings, A and B.</p>
  
  <p>B contains every element in A, and has one additional member, for example:</p>
  
  <pre><code>A = ['dog', 'cat', 'monkey]
  B = ['cat', 'rat', 'dog', 'monkey']
  </code></pre>
  
  <p>I need to write a function to find the extra string in B using <code>O(n)</code>. Please help!</p>
  
  <p>UPDATE: I know the difference can be achieved using PHP in-built function array_diff().</p>