Why is base64_encode() adding a slash “/” in the result?

No. The Base64 alphabet includes A-Z, a-z, 0-9 and + and /.

You can replace them if you don’t care about portability towards other applications.

See: http://en.wikipedia.org/wiki/Base64#Variants_summary_table

You can use something like these to use your own symbols instead (replace - and _ by anything you want, as long as it is not in the base64 base alphabet, of course!).

The following example converts the normal base64 to base64url as specified in RFC 4648:

function base64url_encode($s) {
    return str_replace(array('+', "https://stackoverflow.com/"), array('-', '_'), base64_encode($s));
}

function base64url_decode($s) {
    return base64_decode(str_replace(array('-', '_'), array('+', "https://stackoverflow.com/"), $s));
}