https://en.wikipedia.org/wiki/Base64
The Base64 index table:
| Value | Char | Value | Char | Value | Char | Value | Char | |||
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | A |
16 | Q |
32 | g |
48 | w |
|||
| 1 | B |
17 | R |
33 | h |
49 | x |
|||
| 2 | C |
18 | S |
34 | i |
50 | y |
|||
| 3 | D |
19 | T |
35 | j |
51 | z |
|||
| 4 | E |
20 | U |
36 | k |
52 | 0 |
|||
| 5 | F |
21 | V |
37 | l |
53 | 1 |
|||
| 6 | G |
22 | W |
38 | m |
54 | 2 |
|||
| 7 | H |
23 | X |
39 | n |
55 | 3 |
|||
| 8 | I |
24 | Y |
40 | o |
56 | 4 |
|||
| 9 | J |
25 | Z |
41 | p |
57 | 5 |
|||
| 10 | K |
26 | a |
42 | q |
58 | 6 |
|||
| 11 | L |
27 | b |
43 | r |
59 | 7 |
|||
| 12 | M |
28 | c |
44 | s |
60 | 8 |
|||
| 13 | N |
29 | d |
45 | t |
61 | 9 |
|||
| 14 | O |
30 | e |
46 | u |
62 | + |
|||
| 15 | P |
31 | f |
47 | v |
63 | / |
A quote from Thomas Hobbes‘ Leviathan (be aware of spaces between lines):
Man is distinguished, not only by his reason, but by this singular passion from other animals, which is a lust of the mind, that by a perseverance of delight in the continued and indefatigable generation of knowledge, exceeds the short vehemence of any carnal pleasure.
is represented as a byte sequence of 8-bit-padded ASCII characters encoded in MIME‘s Base64 scheme as follows:
TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlz IHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2Yg dGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGlu dWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRo ZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=
In the above quote, the encoded value of Man is TWFu. Encoded in ASCII, the characters M, a, and n are stored as the bytes 77, 97, and 110, which are the 8-bit binary values 01001101, 01100001, and 01101110. These three values are joined together into a 24-bit string, producing 010011010110000101101110. Groups of 6 bits (6 bits have a maximum of 26 = 64 different binary values) are converted into individual numbers from left to right (in this case, there are four numbers in a 24-bit string), which are then converted into their corresponding Base64 character values.
| Text content | M | a | n | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ASCII | 77 (0x4d) | 97 (0x61) | 110 (0x6e) | |||||||||||||||||||||
| Bit pattern | 0 | 1 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | 1 | 0 |
| Index | 19 | 22 | 5 | 46 | ||||||||||||||||||||
| Base64-encoded | T | W | F | u | ||||||||||||||||||||
As this example illustrates, Base64 encoding converts three octets into four encoded characters.
https://en.wikipedia.org/wiki/MIME#Content-Transfer-Encoding
http://php.net/manual/en/function.base64-encode.php
base64_encode
(PHP 4, PHP 5, PHP 7)
base64_encode — Encodes data with MIME base64
Description?
string base64_encode ( string $data )
Encodes the given data with base64.
This encoding is designed to make binary data survive transport through transport layers that are not 8-bit clean, such as mail bodies.
Base64-encoded data takes about 33% more space than the original data.
https://zh.wikipedia.org/wiki/Base64
在MIME格式的电子邮件中，base64可以用来将binary的字节序列数据编码成ASCII字符序列构成的文本。使用时，在传输编码方式中指定base64。使用的字符包括大小写字母各26个，加上10个数字，和加号“+”，斜杠“/”，一共64个字符，等号“=”用来作为后缀用途。
完整的base64定义可见RFC 1421和RFC 2045。编码后的数据比原始数据略长，为原来的43。在电子邮件中，根据RFC 822规定，每76个字符，还需要加上一个回车换行。可以估算编码后数据长度大约为原长的135.1%。
转换的时候，将三个byte的数据，先后放入一个24bit的缓冲区中，先来的byte占高位。数据不足3byte的话，于缓冲器中剩下的bit用0补足。然后，每次取出6（因为26=64）个bit，按照其值选择ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/中的字符作为编码后的输出。不断进行，直到全部输入数据转换完成。
当原数据长度不是3的整数倍时, 如果最后剩下一个输入数据，在编码结果后加2个“=”；如果最后剩下两个输入数据，编码结果后加1个“=”；如果没有剩下任何数据，就什么都不要加，这样才可以保证数据还原的正确性。
https://en.wikipedia.org/wiki/ASCII
American Standard Code for Information Interchange
Originally based on the English alphabet, ASCII encodes 128 specified characters into seven-bit integers as shown by the ASCII chart on the right.[8] The characters encoded are numbers 0 to 9, lowercase letters a to z, uppercase letters A to Z, basic punctuation symbols, control codes that originated with Teletype machines, and a space. For example, lowercase j would become binary 1101010 and decimal 106. ASCII includes definitions for 128 characters: 33 are non-printing control characters (many now obsolete)[9] that affect how text and space are processed[10] and 95 printable characters, including the space (which is considered an invisible graphic[1]:223[11]).