In PHP 7, the padding issue with base64_decode() is no more - the following is totally fine:

function base64_encode_url($string) {
    return str_replace(['+','/','='], ['-','_',''], base64_encode($string));
}

function base64_decode_url($string) {
    return base64_decode(str_replace(['-','_'], ['+','/'], $string));
}

Checked here with random_bytes() and random lengths:

https://3v4l.org/aEs4o