研究base64_encode的算法

$string = 'abcde';

echo 'origin php 64 encode:';
var_dump(base64_encode($string));

function my_base64_encode($string = "") {
    $string_length = strlen($string);
//看余数
    $mod = $string_length % 3;
//echo 'mod:<br />';
//var_dump($mod);
    $add_bin_value = '';
    switch ($mod) {
        case 1:
            $add_bin_value = "0000000000000000";
            break;
        case 2:
            $add_bin_value = "00000000";
            break;
    }

    $group_count = ceil($string_length / 3); //得到多少组
//echo 'group <br />';
//var_dump($group_count);
    $string_arr = str_split($string, 1);
    $all_bin_value = '';
    foreach ($string_arr as $key => $value) {

        $ascii_int = ord($value); //得到这个字符在ascii表中的对应的10进制整数
        //然后把这个10进制整数,转换为二进制
        //echo '$ascii_int:' . $ascii_int . "<br />";

        $bin_value = decbin($ascii_int); //由于ascii的范围是0-255,如果是84,54这样比较小的值,这个函数会得到是一个1-7位的二进制。为了做到8位。于是增加判断一下,最高位要加0填充
        $str_len = strlen($bin_value);
        if ($str_len < 8) {
            $add_zero_count = 8 - $str_len; //要增加这么多个0在前面,以便补充8位
            for ($i = 0; $i < $add_zero_count; $i++) {
                $bin_value = '0' . $bin_value;
            }
        }
        //echo '$bin_value:' . $bin_value . "<br />";
        $all_bin_value.= $bin_value;
    }
    $all_bin_value = $all_bin_value . $add_bin_value;
    //var_dump($all_bin_value);
    $bin_group_list = str_split($all_bin_value, 6); //每6个二进制一组,进行分组
    //分组结果:
    //var_dump($bin_group_list);
    $base64_char = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
    $base64_char_arr = str_split($base64_char, 1);
//var_dump($base64_char_arr);
    $dec_int = 0;
    $base64_encode_result = '';
    foreach ($bin_group_list as $key => $value) {
        if ($value == '000000') {
            $base64_encode_result.='=';
        } else {
            $dec_int = bindec($value); //将二进制转换为10进制整数
            //echo $dec_int . "<br />";
            $base64_encode_result .= $base64_char_arr[$dec_int];
        }
    }
    
    return $base64_encode_result;
}


$base64_encode_result = my_base64_encode($string);
echo 'encode result:';
var_dump($base64_encode_result);