LCDUNG.

/**
   * Interpolates context values into the message placeholders.
   */
  function interpolate($message, array $context = array())
  {
      // build a replacement array with braces around the context keys
      $replace = array();
      foreach ($context as $key => $val) {
          // check that the value can be casted to string
          if (!is_array($val) && (!is_object($val) || method_exists($val, '__toString'))) {
              $replace['{' . $key . '}'] = $val;
          }
      }

      // interpolate replacement values into the message and return
      return strtr($message, $replace);
  }

  // a message with brace-delimited placeholder names
  $message = "User {username} created";

  // a context array of placeholder names => replacement values
  $context = array('username' => 'bolivar');

  // echoes "User bolivar created"
  echo interpolate($message, $context);