+ means a space only in application/x-www-form-urlencoded content, such as the query part of a URL:

http://www.example.com/path/foo+bar/path?query+name=query+value

In this URL, the parameter name is query name with a space and the value is query value with a space, but the folder name in the path is literally foo+bar, not foo bar.

%20 is a valid way to encode a space in either of these contexts. So if you need to URL-encode a string for inclusion in part of a URL, it is always safe to replace spaces with %20 and pluses with %2B. This is what eg. encodeURIComponent() does in JavaScript. Unfortunately it‘s not what urlencode does in PHP (rawurlencode is safer).

See Also HTML 4.01 Specification application/x-www-form-urlencoded