<?php

function my_base64_encode($str)
{
    $base64 = str_split('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/');
    $bit_pattern = '';
    $padding = 0;
    $encoded = '';

    foreach (str_split($str) as $char) {
        $bit_pattern .= sprintf('%08s', decbin(ord($char)));
    }

    $bit_pattern = str_split($bit_pattern, 6);

    $offset = count($bit_pattern) - 1;
    if (strlen($bit_pattern[$offset]) < 6) {
        $padding = 6 - strlen($bit_pattern[$offset]);
        $bit_pattern[$offset] .= str_repeat('0', $padding);
        $padding /= 2;
    }

    foreach ($bit_pattern as $bit6) {
        $index = bindec($bit6);
        $encoded .= $base64[$index];
    }

    return $encoded.str_repeat('=', $padding);
}

function my_base64_decode($str)
{
    $base64 = str_split('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/');
    $bit_pattern = '';
    $padding = substr_count(substr(strrev($str), 0, 2), '=');
    $decoded = '';

    foreach (str_split($str) as $b64_encoded) {
        $bit_pattern .= sprintf('%06s', decbin(array_search($b64_encoded, $base64)));
    }

    $bit_pattern = str_split($bit_pattern, 8);
    if ($padding) {
        $bit_pattern = array_slice($bit_pattern, 0, -$padding);
    }

    foreach ($bit_pattern as $bin) {
        $decoded .= chr(bindec($bin));
    }

    return $decoded;
}

$to_encode = 'Hello World ! :)';
$base64_encode = base64_encode($to_encode);
$my_base64_encode = my_base64_encode($to_encode);
$base64_decode = base64_decode($base64_encode);
$my_base64_decode = my_base64_decode($my_base64_encode);

var_dump(
    'To encode/decode : '.$to_encode,
    '-------------------------------------------',
    'base64_encode    : '.$base64_encode,
    'my_base64_encode : '.$my_base64_encode,
    '-------------------------------------------',
    'base64_decode    : '.$base64_decode,
    'my_base64_decode : '.$my_base64_decode
);