Change single variable value in querystring [closed]

Use

• parse_url() to extract the query string from the URL

• parse_str() to split the query string into an array

• array_merge() to add a new array "b" => 5

• http_build_query() to re-build a query string

• The remaining parts from the first step (protocol, host, path…) to re-build the full URL or – if you have the HTTP pecl extension – a http_build_url() with HTTP_URL_JOIN_QUERY will alleviate much of the work.