Check if a string seems to be base64 encoded

is_base64.php

<?php
/**
 * @param   string  $str    
 * @return  bool
 */
function is_base64($str)
{
    return (bool)preg_match('`^[a-zA-Z0-9+/]+={0,2}$`', $str);
}

None of the answers worked for me. In my case it was the lack of spaces that I used to tell if it was base64 encoded
$teststring = substr($string,4,100);
$not64 = strpos($teststring," ");
It's not in a function, but it has worked for me. I'm sure people will tell me that it won't always work, but ALL of the functions suggested above returned false for me regardless of the string.

A slightly adapted version incase anyone is validating base64 encoded files (not fully tested):

private function isBase64Encoded(string $s): bool
  {
    if ((bool) preg_match('/^[a-zA-Z0-9\/\r\n+]*={0,2}$/', $s) === false) {
      return false;
    }

    $decoded = base64_decode($s, true);
    if ($decoded === false) {
      return false;
    }

    $mimeType = finfo_buffer(finfo_open(), $decoded, FILEINFO_MIME_TYPE);

    return $decoded !== false && base64_encode($decoded) === $s && $mimeType;
  }