title: What does the function _($string) do in PHP?
body: <p>I noticed this piece of code in Wordpress 2.9.1 (/wp-includes/compat.php), I don't understand it:</p>
  
  <pre><code>if ( !function_exists('_') ) {
    function _($string) {
      return $string;
    }
  }
  </code></pre>
  
  <p>It seems that PHP indeed has a function <code>_($string)</code> but I can't find the documentation for it.</p>