I must be missing some basic thing about cookies. On localhost, when I set a cookie on server side and specify the domain explicitly as localhost (or .localhost). t
None of the answers here worked for me. I fixed it by putting my PHP as the very very first thing in the page.
Like other headers, cookies must be sent before any output from your script (this is a protocol restriction). This requires that you place calls to this function prior to any output, including and tags as well as any whitespace.
From http://php.net/manual/en/function.setcookie.php