title: array_filter based on keys from another array
body: <p>I have two arrays:</p>
  
  <p><code>$arr1 = array('a' =&gt; 10, 'b' =&gt; 20);</code></p>
  
  <p><code>$arr2 = array('a' =&gt; 10, 'b' =&gt; 20, 'c' =&gt; 30);</code></p>
  
  <p>How can I use array_filter to drop elements from <code>$arr2</code> that don't exist in <code>$arr1</code> ? Like "c" in my example...</p>