src/CoreBundle/Contao/Hooks/AbstractContentElementAndModuleCallback.php
public function loadCallback(string $value = null)
    {
        return null === $value ? null : trim(\base64_encode($value), '=');
    }
public function loadCallback(string $value = null)
    {
        return null === $value ? null : trim(\base64_encode($value), '=');
    }
Base64 encode.

@param string|null $value The value.

@return string|null