Developing a tracking pixel

Question

I am trying to build a pixel that would track the current URL the user is on when they visit. I can use either JS (preferred) or a 1x1 image pixel. With JS I am assuming tha

Answer 1

Output 1px x 1px this way:

header('Content-type: image/png');
echo gzinflate(base64_decode('6wzwc+flkuJiYGDg9fRwCQLSjCDMwQQkJ5QH3wNSbCVBfsEMYJC3jH0ikOLxdHEMqZiTnJCQAOSxMDB+E7cIBcl7uvq5rHNKaAIA'));