Detailed explanation of the usage of pack and unpack functions in PHP

There are two functions pack and unpack in PHP. Many PHPers have never used them in actual projects, and they don’t even know what these two methods are used for. This article will introduce to you what they are used for.

pack

string pack ( string $format [, mixed $args [, mixed $... ]] )

This function is used to pack the corresponding parameters ($args) into a binary string.

The first parameter, $format, has the following options (there are many optional parameters, and I will select a few common ones to explain later):

After looking at so many parameters, I was really confused for the first time. Most of the instructions are easy to understand, but what is the byte order of host, big endian, little endian, etc.? The following content is relatively boring, but it must be understood, so stick with it.

What is the byte order?

is the order of bytes. To put it bluntly, it is the order in which multi-byte data is stored (one byte obviously does not require order).

For example, the corresponding binary representations of A and B are 0100 0001 and 0100 0010 respectively. For storing the string AB, we can either 0100 0001 0100 0010 or 0100 0010 0100 0001. This order is the so-called byte order.

High/Low Bytes

For example, the string AB, the left is high and the right is low (our normal reading order), A is the high word section, B is the low byte

High/low address

Assuming that 0x123456 is stored in order from high to low, the memory is Stored like this:

High address-> Low address

12 -> 34 -> 56

Big endian byte order ( Network byte order)

Big endian means placing the high-order bytes at the low address end of the memory and the low-order bytes at the high address end. In network transmission (such as TCP/IP), the low address end (high byte) is placed at the beginning of the stream. For a 2-byte string (AB), the transmission order is: A (0-7bit), B (8- 15bit).

Then little-endian byte order is naturally opposite to big-endian.

Host byte order

Indicates the byte order of the machine that year (that is, the network byte order is determined, and the host byte order The order is determined by the machine), generally little-endian.

a and A (packed string, filled with NUL or spaces)

$string = pack('a6', 'china’);
var_dump($string); //输出结果: string(6) "china",最后一个字节是不可见的NUL
echo ord($string[5]); //输出结果: 0(ASCII码中0对应的就是nul)

//A同理
$string = pack('A6', 'china’);
var_dump($string); //输出结果: string(6) "china ",最后一个字节是空格
echo ord($string[5]); //输出结果: 32(ASCII码中32对应的就是空格)

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Comes with an ASCII table (you can use man ascii to view it under linux/unix)

h and H

$string = pack('H3', 281);
var_dump($string); //输出结果: string(2) “(“
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出结果: 40 16

h and H need special explanation. They treat the corresponding parameters as hexadecimal characters and then package them. What does that mean? For example, the above 281 will be converted to 0x281 before packaging, because one hexadecimal digit corresponds to four binary digits. The above 0x281 is only 1.5 bytes, and will be supplemented with 0 by default to become 0x2810, the decimal corresponding to 0x28. is 40((), the decimal corresponding to 0x10 is 16 (dle invisible character).

c and C

$string = pack('c3', 67, 68, -1);
var_dump($string); //输出:string(3) “CD�”
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 67 68 225

The final output should be I think 67 68 -1

ord obtains the ASCII code of the character (range 0-255). At this time, the character corresponding to -1 (0000 0001) will be output in the form of complement, which is 255 (1111 1110 0000 0001 = 1111 1111)

Integer related

All integer types are used exactly the same way, mainly pay attention to their bit sums Endianness is sufficient. L is used as an example below:

$string = pack('L', 123456789);
var_dump($string); //输出:string(4) “�[“

for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 21 205 91 7

f and d

$string = pack('f', 12345.123);
var_dump($string);
//输出:string(4) "~�@F”
var_dump(unpack('f', $string)); //这里提前用到了unpack，后面会讲解
//输出:float(12345.123046875)

f and d are for floating point packaging , as for why it is 12345.123 before packaging and 12345.123046875 after unpacking, this has something to do with the storage of floating point numbers.

x, X, Z, @

$string = pack('x'); //打包一个nul字符串
echo ord($string); //输出: 0

Regarding X (capital

$string = pack('Z2', 'abc5'); //其实就是将从Z后面的数字位置开始，全部设置为nul
var_dump($string); //输出:string(2) “a”
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 97 0

The use of unpack is quite simple. It just talks about unpacking the data packed by pack. What parameters are used when packaging are used to unpack. I am too lazy to explain the specific use. I will list a few small ones. Example:

$string = pack('@4'); //我理解为填充N个nul
var_dump($string); //输出: string(4) “”
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 0 0 0 0

What are the uses of these two functions

·

Data communication (Communicate with other languages ​​through binary format)

·

Data encryption (If you don’t tell the third party your packaging method, it will be relatively difficult for the other party to unpack it. )

·Save space (for example, storing relatively large numbers as strings will waste a lot of space, and only 4 digits are needed to package them into binary format)

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This article is reproduced from: https://www.jianshu.com/p/942907071a4c

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