Extracting a certain section of a string

How do I get the value of tracking_id from a string similar to the string below:

action_type=PAY&transaction_type=Adaptive+Payment+PAY&tracking_id=bovxedxgM开发者_开发百科&status=COMPLETED&

That is just a snipped of a much longer string.

Use parse_str():

parse_str('your-string-here', $data);
echo $data['tracking_id'];

$arguments = array();
parse_str('action_type=PAY&transaction_type=Adaptive+Payment+PAY&tracking_id=bovxedxgM&status=COMPLETED', $arguments);
$tracking_id = $arguments['tracking_id'];

http://php.net/parse-str