title: Multidimensional PHP array - Key exists
body: <p>I want a function that will return TRUE or FALSE based on if a given key exists in a multidimensional array in PHP.</p>
  
  <p>I haven't been able to figure out a recursive function to perform this action.</p>
  
  <p>A sample of what this could do:</p>
  
  <pre><code>$array = array(
      'key 1' =&gt; array(
          'key 1.1' =&gt; array()
          'key 1.2' =&gt; array()
      ),
      'key 2' =&gt; array(
          'key 2.1' =&gt; array(
              'key 2.1.1' =&gt; array()
          )
          'key 2.2' =&gt; array()
      )
  );
  
  multi_array_key_exists('key 1', $array); // return TRUE
  
  multi_array_key_exists('key 2.1.1', $array); // return TRUE
  
  multi_array_key_exists('key 3', $array); // return FALSE
  </code></pre>