Replace URL strings in PHP

[alltech]

I have within cdata tags URL for example it would be www.thrifty.co.uk/cgi-bin....etc but when selecting the 'book' button i have it just gives blank page. Please help!!! Code shown below

foreach($xmlDoc->car as $car)
{
$url = (string)$car->book;
$url = str_replace('wheels4rent.net','$url');

echo "<tr><td width=200px><img src='".$car->carimage."' align='right' style='padding:1px; width:100px'><b>".$car->cartype."</b><br>".$car->carsipp."<br>".$car->transmission."</td><td><b>".$car->carexample."</b></td><td><b>&pound;".$car->price."
</b><br>Unlimited Miles</b><br>
<input type=button onclick='javascript:window.open($url);' value='Book Now'></td></tr>";


}

[Dormilich]

variables ain’t resolved in single quotes (line #4).

and in JavaScript, string values need to be quoted (line #8).