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

Question

I am encoding the URL suffix of my application:

$url = 'subjects?_d=1'; echo base64_encode($url);  // Outputs c3ViamVjdHM/X2Q9MQ== 

Notice the slash before 'X2'.

Why is this happening? I thought base64 only outputted A-Z, 0-9 and '=' as padding?

Answer 1

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($s) {     return str_replace(array('+', '/'), array('-', '_'), base64_encode($s)); }  function base64url_decode($s) {     return base64_decode(str_replace(array('-', '_'), array('+', '/'), $s)); } 

Answer 2

In addition to all of the answers above, pointing out that / is part of the expected base64 alphabet, it should be noted that the particular reason you saw a / in your encoded string, is because when base64 encoding ASCII text, the only way to generate a / is to have a question mark in a position divisible by three.