一、Base64编码原理

步骤1：将所有字符转化为ASCII码；
步骤2：将ASCII码转化为8位二进制；
步骤3：将二进制分组，每组6位， 组数需3的倍数，否则后面补全是0的组；
步骤4：将每组6位二进制前补两个0凑足8位，再转为十进制；
步骤5：从Base64编码表获取十进制对应的Base64编码；



说明：不足补0的最终Base64编码为=，而不是A

二、Base64编码示例

示例一：zyq对应Base64编码为enlx


示例二：mzwu对应Base64编码为bXp3dQ==

例二中, 被编码字符长度不是3的倍数，后面都用0代替，对应的输出字符为=

/**
 * 编码
 * 
 * @param string $string
 * @return
 */
function b64encode($string = '')
{
    $binval = convertToBinary($string);

    $final = "";
    $start = 0;
    while ($start < strlen($binval)) {
        if (strlen(substr($binval, $start)) < 6)
            $binval .= str_repeat("0", 6 - strlen(substr($binval, $start)));
        $tmp = bindec(substr($binval, $start, 6));
        if ($tmp < 26)
            $final .= chr($tmp + 65);
        elseif ($tmp > 25 && $tmp < 52)
            $final .= chr($tmp + 71);
        elseif ($tmp == 62)
            $final .= "+";
        elseif ($tmp == 63)
            $final .= "/";
        elseif (!$tmp)
            $final .= "A";
        else
            $final .= chr($tmp - 4);
        $start += 6;
    }
    if (strlen($final) % 4 > 0)
        $final .= str_repeat("=", 4 - strlen($final) % 4);
    return $final;
}

/**
 * 转成二进制
 * 
 * @param mixed $string
 * @return
 */
function convertToBinary($string)
{
    if (strlen($string) <= 0)
        return;
    $tmp = decbin(ord($string[0]));
    $tmp = str_repeat("0", 8 - strlen($tmp)) . $tmp;
    return $tmp . convertToBinary(substr($string, 1));
}

/**
 * 解码
 * 
 * @param mixed $str
 * @return
 */
function b64decode($str)
{
    $B64List = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
    $len = strlen($str);
    if ($len % 4 != 0)
        return '';
    $res = $bins = '';
    for ($i = 0; $i < $len; $i++) {
        $nChar = substr($str, $i, 1);
        if ($nChar == '=')
            break;
        $oldValue = strpos($B64List, $nChar);
        $binValue = substr('000000' . decbin($oldValue), -6);
        $bins .= $binValue;

        if (strlen($bins) >= 8) {
            $deChar = substr($bins, 0, 8);
            $bins = substr($bins, 8);
            $res .= chr(bindec($deChar));
        }
    }
    return $res;
}