Manipulate a url string by adding GET parameters

Question

I want to add GET parameters to URLs that may and may not contain GET parameters without repeating ? or &.

Example:

If I want t

Answer 1

Here's a shorter version of the accepted answer:

$url .= (parse_url($url, PHP_URL_QUERY) ? '&' : '?') . 'category=action';

Edit: as discussed in the accepted answer, this is flawed in that it doesn't check to see if category already exists. A better solution would be to treat the $_GET for what it is - an array - and use functions like in_array().