You can use a combination of regexp and the explode function. Please note that this is dependent upon the actual coding of the page you are requesting with cURL, as I am looking at the values being exploded by two returns - \n\n
$curlPage = "21 / BTC \n13.22000001\n\n42 / BTC \n112.10000000";
$arr = explode("\n\n", $curlPage);
foreach($arr as $key)
{
list($top, $bottom) = explode("\n", $key);
$top = preg_replace("/[^0-9]/", "", $top);
if($top == "21") echo($bottom);
}
The first line shows what my value for $curlPage is (for testing it). This should be equal to the contents of the page (provided the contents are what you posted and each section is separated by two line breaks (\n\n)
The two important variables in the foreach() statement is $top and $bottom. $top will be equal to the number, like 21. And the $bottom variable will be equal to the bottom number like 13.22000001