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

MOST ANSWERS HERE ARE NOT RELIABLE

In fact, there is no reliable answer, as many non-base64-encoded text will be readable as base64-encoded, so there's no default way to know for sure.

Further, it's worth noting that base64_decode will decode many invalid strings For exmaple, and is not valid base64 encoding, but base64_decode WILL decode it. As jw specifically. (I learned this the hard way)

That said, your most reliable method is, if you control the input, to add an identifier to the string after you encode it that is unique and not base64, and include it along with other checks. It's not bullet-proof, but it's a lot more bullet resistant than any other solution I've seen. For example:

function my_base64_encode($string){
  $prefix = 'z64ENCODEDz_';
  $suffix = '_z64ENCODEDz';
  return $prefix . base64_encode($string) . $suffix;
}

function my_base64_decode($string){
  $prefix = 'z64ENCODEDz_';
  $suffix = '_z64ENCODEDz';
  if (substr($string, 0, strlen($prefix)) == $prefix) {
    $string = substr($string, strlen($prefix));
  }
  if (substr($string, (0-(strlen($suffix)))) == $suffix) {
    $string = substr($string, 0, (0-(strlen($suffix))));
  }
      return base64_decode($string);
}

function is_my_base64_encoded($string){
  $prefix = 'z64ENCODEDz_';
  $suffix = '_z64ENCODEDz';
  if (strpos($string, 0, 12) == $prefix && strpos($string, -1, 12) == $suffix && my_base64_encode(my_base64_decode($string)) == $string && strlen($string)%4 == 0){
    return true;
  } else {
    return false;
  }
}