OpenPGP, described in RFC 4880, describes Radix-64 encoding, also known as "ASCII Armor". Radix-64 is identical to the "Base64" encoding described from MIME, with the addition of an optional 24-bit CRC. The checksum is calculated on the input data before encoding; the checksum is then encoded with the same Base64 algorithm and, using an additional "=" symbol as separator, appended to the encoded output data.

composer require bigwhoop/radix64

<?php
use Bigwhoop\Radix64\Radix64;
use Bigwhoop\Radix64\ChecksumMismatchException;

$output = Radix64::encode('this is a message');

// dGhpcyBpcyBhIG1lc3NhZ2U=
// =NTMy

try {
    $input = Radix64::decode($output);
} catch (ChecksumMismatchException $e) {
    // Oops ...
}

composer install --dev
vendor/bin/phpunit