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);
}

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;
  }