title: Change array keys to lowercase
body: <p>I'm looking for a nifty php solution to replace a standard forloop. i dont care at all about the inner workings of my normalize method.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>$pairs=[];
  foreach($items as $key =&gt; $val){
      $key = $this-&gt;normalizeKey($key);
      $pairs[$key] = $val;
  }
  
  private function normalizeKey($key)
  {
      // lowercase string
      $key = strtolower($key);
  
      // return normalized key
      return $key;
  }   
  </code></pre>
  
  <p>I'd like to learn the PHP language a bit better. I thought <code>array_walk</code> would be nice to use, but that operates on the values and not the keys. </p>
  
  <p>I am looking for a PHP array method that can perform this code instead of a <code>foreach</code> loop.</p>