How to get and change URL variable PHP

Question

Hi could anyone help me with this I have a URL like

parent/child/a=1&b=2$c=3

then I have a link that would add variable to that URL

Answer 1

$query = $_GET;
// replace parameter(s)
$query['d'] = 'new_value';
// rebuild url
$query_result = http_build_query($query);
// new link
Link