Fatal error: Constant expression contains invalid operations почему?

<?php
class Pars {

    public $id;
    private $url = 'http://site.ru/str.php?id='.$id;

    function __construct ($id) {
        $get = file_get_contents ($url);
        $get = strip_tags ($get);
        $get = explode ('<br />', $get);
        var_dump ($get);
    }
}

?>