Use parse_str()
function (https://www.geeksforgeeks.org/php-parse_str-function/)
$str = 'search=test&site=1&salesperson=2&referral=6&product=10&estimate=1000&sort=date&open=on&filter_sbmt=Filter+Prospect&limit=30';
parse_str($str);
echo $search; // 'test'
echo $site; // '1'
Don't try to make this call without storing the result on an array, this is discouraged and deprecated as of PHP 7.2 (https://www.php.net/manual/en/function.parse-str.php)
You have then to pass a result object to store the results on as a second parameter:
$str = 'search=test&site=1&salesperson=2&referral=6&product=10&estimate=1000&sort=date&open=on&filter_sbmt=Filter+Prospect&limit=30';
$output = array();
parse_str($str, $output);
print_r($output); // Array ( [search] => test [site] => 1 ...)
echo $output['search']; // 'test'
echo $output['site']; // '1'