/**
* 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) {
      $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);