title: Named backreferences with preg_replace
body: <p>Pretty straightforward; I can't seem to find anything definitive regarding PHP's <code>preg_replace()</code> supporting named backreferences:</p>
  
  <pre><code>// should match, replace, and output: user/profile/foo
  $string = 'user/foo';
  echo preg_replace('#^user/(?P&lt;id&gt;[^/]+)$#Di', 'user/profile/(?P=id)', $string);
  </code></pre>
  
  <p>This is a trivial example, but I'm wondering if this syntax, <code>(?P=name)</code> is simply not supported. Syntactical issue, or non-existent functionality?</p>