How to check if a string is base64 valid in PHP

Question

I have a string and want to test using PHP if it\'s a valid base64 encoded or not.

Answer 1

i know that i resort a very old question, and i tried all of the methods proposed; i finally end up with this regex that cover almost all of my cases:

$decoded = base64_decode($string, true);
if (0 < preg_match('/((?![[:graph:]])(?!\s)(?!\p{L}))./', $decoded, $matched)) return false;

basically i check for every character that is not printable (:graph:) is not a space or tab (\s) and is not a unicode letter (all accent ex: èéùìà etc.)

i still get false positive with this chars: £§° but i never use them in a string and for me is perfectly fine to invalidate them. I aggregate this check with the function proposed by @merlucin

so the result:

function is_base64($s)
{
  // Check if there are valid base64 characters
  if (!preg_match('/^[a-zA-Z0-9\/\r\n+]*={0,2}$/', $s)) return false;

  // Decode the string in strict mode and check the results
  $decoded = base64_decode($s, true);
  if(false === $decoded) return false;

  // if string returned contains not printable chars
  if (0 < preg_match('/((?![[:graph:]])(?!\s)(?!\p{L}))./', $decoded, $matched)) return false;

  // Encode the string again
  if(base64_encode($decoded) != $s) return false;

  return true;
}