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A brief discussion on how to use pack and unpack in PHP

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May 18, 2018 pm 01:59 PM

Instructions

This time I will bring you a detailed explanation of how to use pack and unpack in PHP, and a brief discussion of the precautions for using pack and unpack in PHP. The following is a practical case, let's take a look.

pack

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

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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):

##NUnsigned long (32 bits, big endian)VUnsigned long integer (32 bits, little endian)qSigned long integer (64 bits, host byte order)QUnsigned long Long integer type (64 bits, host byte order)JUnsigned long integer type (64 bits, big endian byte order)PUnsigned long integer (64 bits, little endian)fSingle PrecisiondDouble precision floating point type(machine dependent size)xNUL byteXRoll back one byteZ Padding string blanks with NUL bytes (new in PHP 5.5)@NUL padding to absolute position

这么多参数看下来，我第一次是真心懵逼了，大部分说明都很好理解，但是其中的主机、大端、小端等字节序是什么鬼呢？接下里的内容比较枯燥，但必须理解才行，坚持吧。

字节序是什么？

就是字节的顺序，说白了就是多字节数据的存放顺序(一个字节显然不需要顺序)。

比如A和B分别对应的二进制表示为0100 0001、0100 0010。对于储存字符串AB，我们可以0100 0001 0100 0010也可以0100 0010 0100 0001，这个顺序就是所谓的字节序。

高/低位字节

比如字符串AB，左高右低(我们正常的阅读顺序)，A为高字节，B为低字节

高/低地址

假设0x123456是按从高位到底位的顺序储存，内存中是这样存放的：

高地址 -> 低地址
12 -> 34 -> 56

大端字节序(网络字节序)

大端就是将高位字节放到内存的低地址端，低位字节放到高地址端。网络传输中(比如TCP/IP)低地址端(高位字节)放在流的开始，对于2个字节的字符串(AB)，传输顺序为：A(0-7bit)、B(8-15bit)。

那么小端字节序自然和大端相反。

主机字节序

表示当年机器的字节序(也就是网络字节序是确定的，而主机字节序是依机器确定的)，一般为小端字节序。

a和A(打包字符串，用NUL或者空格填充)

$string = pack('a6', 'china');
var_dump($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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附赠ASCII表一张(linux/unix下可以使用man ascii查看)

h和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

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h和H需要特殊说明一下，它们是将对应的参数看做十六进制字符然后打包。什么意思呢？比如上面的281，打包前会将281转换为0x281，因为十六进制的一位对应二进制的四位，上面的0x281只有1.5个字节，后面会默认补0变成0x2810，0x28对应的十进制为40(()，0x10对应的十进制为16(dle不可见字符)，懂了吧？不懂可以给我留言。。

c和C

$string = pack(&#39;c3&#39;, 67, 68, -1);
var_dump($string); //输出:string(3) "CD�"
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 67 68 225

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最后输出本能应该觉得是67 68 -1

ord获取的是字符的ASCII码(范围0-255)，这时-1(0000 0001)对应的字符将以补码的形式输出也就是255(1111 1110 + 0000 0001 = 1111 1111)

整型相关

所有的整型类型使用方法完全一样，主要注意它们的位和字节序就可以了，下面以L作为例子展示

$string = pack(&#39;L&#39;, 123456789);
var_dump($string); //输出:string(4) "�["
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 21 205 91 7

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f和d

$string = pack(&#39;f&#39;, 12345.123);
var_dump($string);
//输出:string(4) "~�@F"
var_dump(unpack(&#39;f&#39;, $string)); //这里提前用到了unpack，后面会讲解
//输出:float(12345.123046875)

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f和d是针对浮点数打包，至于为什么打包前是12345.123解包后是12345.123046875，这个和浮点数的储存有关系，后面可以单开一个文章讲解一下IEEE标准

x、X、Z、@

$string = pack(&#39;x&#39;); //打包一个nul字符串
echo ord($string); //输出: 0

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关于X(大写X)，试了N次，没搞明白怎么用，有清楚的童鞋可以给我留言，多谢。

$string = pack(&#39;Z2&#39;, &#39;abc5&#39;); //其实就是将从Z后面的数字位置开始，全部设置为nul
var_dump($string); //输出:string(2) "a"
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 97 0

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$string = pack(&#39;@4&#39;); //我理解为填充N个nul
var_dump($string); //输出: string(4) ""
for($i=0;$i<strlen($string);$i++) {
echo ord($string[$i]) . PHP_EOL;
}
//输出: 0 0 0 0

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unpack

array unpack ( string $format , string $data )

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unpack的使用相当简单，就是讲pack打包的数据解包，打包的时候用的什么参数，就用什么参数解包，具体使用懒得说了，列几个小例子

$string = pack(&#39;L4&#39;, 1, 2, 3, 4);
var_dump(unpack(&#39;L4&#39;, $string));
//输出:
array(4) {
[1]=>
int(1)
[2]=>
int(2)
[3]=>
int(3)
[4]=>
int(4)
}
$string = pack('L4', 1, 2, 3, 4);
var_dump(unpack('Ll1/Ll2/Ll3/Ll4', $string)); //可以指定key，用/分割
//输出:
array(4) {
["l1"]=>
int(1)
["l2"]=>
int(2)
["l3"]=>
int(3)
["l4"]=>
int(4)
}

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这两个函数到底有啥用途

数据通信(通过二进制格式与其它语言通信)
数据加密(如果不告诉第三方你的打包方式，对方解包的难度就相对很大)
节省空间(比如比较大的数字按字符串储存会浪费很多空间，打包成二进制格式才需要4位<32位数字>)

相信看了本文案例你已经掌握了方法，更多精彩请关注php中文网其它相关文章！

Code	Description
a	Padding string gaps with NUL bytes
A	Fill the string with SPACE(space)
h	Hexadecimal string, low digit first
H	Hexadecimal string, high-order bit first
c	Signed characters
C	Unsigned character
s	Signed short (16 bits, host byte order)
S	Unsigned short (16 bits, host byte order)
n	None Signed short (16 bits, big endian)
v	Unsigned short (16 bits, little endian)
i	Signed integer (machine dependent size endian)
I	Unsigned Integer type (machine dependent size byte order)
l	Signed long integer type (32 bits, host byte order)
L	Unsigned long (32-bit, host byte order)






	Floating point type(machine dependent size)

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