title: PHP add value to existing array value
body: <p>I currently have two arrays, the key in each is a product id, whilst the value is the quantity:</p>
  
  <pre><code>$array_1 = ([10] =&gt; 1, [13] =&gt; 3, [27] =&gt; 1, [32] =&gt; 2);
  $array_2 = ([8] =&gt; 1, [10] =&gt; 1, [12] =&gt; 1, [20] =&gt; 2, [27] =&gt; 1);
  </code></pre>
  
  <p>I want to combine the two array, but add the values together where the keys match.  Is there a method to do this without iterating through the arrays? </p>