title: Fastest way to add prefix to array keys?
body: <p>What is the fastest way to add string prefixes to array keys?</p>
  
  <p>Input</p>
  
  <pre><code>$array = array(
   '1' =&gt; 'val1',
   '2' =&gt; 'val2',
  );
  </code></pre>
  
  <p>Needed output:</p>
  
  <pre><code>$array = array(
    'prefix1' =&gt; 'val1',
    'prefix2' =&gt; 'val2',
  );
  </code></pre>