计算机中的字符表示都是2进制，1B（字节）=8bit 用八个内存单元为一个位标号（编码），这是ASCII编码 8bit用了7bit 2^7=128个 扩展的ASCII用了8bit 2^8=256个
256对汉字是远远不够，代替汉字就要用多位表示，gb2312是2字节 utf8是3字节
1、ASCII表中的字符分 为可打印与非打印字符
非打印字符 ： 0-31 + 127
剩下的是可打印字符
ASCII码表
| Bin | Dec | Hex | 缩写/字符 | 解释 |
| 00000000 | 0 | 00 | NUL(null) | 空字符 |
| 00000001 | 1 | 01 | SOH(start of headling) | 标题开始 |
| 00000010 | 2 | 02 | STX (start of text) | 正文开始 |
| 00000011 | 3 | 03 | ETX (end of text) | 正文结束 |
| 00000100 | 4 | 04 | EOT (end of transmission) | 传输结束 |
| 00000101 | 5 | 05 | ENQ (enquiry) | 请求 |
| 00000110 | 6 | 06 | ACK (acknowledge) | 收到通知 |
| 00000111 | 7 | 07 | BEL (bell) | 响铃 |
| 00001000 | 8 | 08 | BS (backspace) | 退格 |
| 00001001 | 9 | 09 | HT (horizontal tab) | 水平制表符 |
| 00001010 | 10 | 0A | LF (NL line feed, new line) | 换行键 |
| 00001011 | 11 | 0B | VT (vertical tab) | 垂直制表符 |
| 00001100 | 12 | 0C | FF (NP form feed, new page) | 换页键 |
| 00001101 | 13 | 0D | CR (carriage return) | 回车键 |
| 00001110 | 14 | 0E | SO (shift out) | 不用切换 |
| 00001111 | 15 | 0F | SI (shift in) | 启用切换 |
| 00010000 | 16 | 10 | DLE (data link escape) | 数据链路转义 |
| 00010001 | 17 | 11 | DC1 (device control 1) | 设备控制1 |
| 00010010 | 18 | 12 | DC2 (device control 2) | 设备控制2 |
| 00010011 | 19 | 13 | DC3 (device control 3) | 设备控制3 |
| 00010100 | 20 | 14 | DC4 (device control 4) | 设备控制4 |
| 00010101 | 21 | 15 | NAK (negative acknowledge) | 拒绝接收 |
| 00010110 | 22 | 16 | SYN (synchronous idle) | 同步空闲 |
| 00010111 | 23 | 17 | ETB (end of trans. block) | 传输块结束 |
| 00011000 | 24 | 18 | CAN (cancel) | 取消 |
| 00011001 | 25 | 19 | EM (end of medium) | 介质中断 |
| 00011010 | 26 | 1A | SUB (substitute) | 替补 |
| 00011011 | 27 | 1B | ESC (escape) | 溢出 |
| 00011100 | 28 | 1C | FS (file separator) | 文件分割符 |
| 00011101 | 29 | 1D | GS (group separator) | 分组符 |
| 00011110 | 30 | 1E | RS (record separator) | 记录分离符 |
| 00011111 | 31 | 1F | US (unit separator) | 单元分隔符 |
| 00100000 | 32 | 20 | (space) | 空格 |
| 00100001 | 33 | 21 | ! | |
| 00100010 | 34 | 22 | " | |
| 00100011 | 35 | 23 | # | |
| 00100100 | 36 | 24 | $ | |
| 00100101 | 37 | 25 | % | |
| 00100110 | 38 | 26 | & | |
| 00100111 | 39 | 27 | ' | |
| 00101000 | 40 | 28 | ( | |
| 00101001 | 41 | 29 | ) | |
| 00101010 | 42 | 2A | * | |
| 00101011 | 43 | 2B | + | |
| 00101100 | 44 | 2C | , | |
| 00101101 | 45 | 2D | - | |
| 00101110 | 46 | 2E | . | |
| 00101111 | 47 | 2F | / | |
| 00110000 | 48 | 30 | 0 | |
| 00110001 | 49 | 31 | 1 | |
| 00110010 | 50 | 32 | 2 | |
| 00110011 | 51 | 33 | 3 | |
| 00110100 | 52 | 34 | 4 | |
| 00110101 | 53 | 35 | 5 | |
| 00110110 | 54 | 36 | 6 | |
| 00110111 | 55 | 37 | 7 | |
| 00111000 | 56 | 38 | 8 | |
| 00111001 | 57 | 39 | 9 | |
| 00111010 | 58 | 3A | : | |
| 00111011 | 59 | 3B | ; | |
| 00111100 | 60 | 3C | < | |
| 00111101 | 61 | 3D | = | |
| 00111110 | 62 | 3E | > | |
| 00111111 | 63 | 3F | ? | |
| 01000000 | 64 | 40 | @ | |
| 01000001 | 65 | 41 | A | |
| 01000010 | 66 | 42 | B | |
| 01000011 | 67 | 43 | C | |
| 01000100 | 68 | 44 | D | |
| 01000101 | 69 | 45 | E | |
| 01000110 | 70 | 46 | F | |
| 01000111 | 71 | 47 | G | |
| 01001000 | 72 | 48 | H | |
| 01001001 | 73 | 49 | I | |
| 01001010 | 74 | 4A | J | |
| 01001011 | 75 | 4B | K | |
| 01001100 | 76 | 4C | L | |
| 01001101 | 77 | 4D | M | |
| 01001110 | 78 | 4E | N | |
| 01001111 | 79 | 4F | O | |
| 01010000 | 80 | 50 | P | |
| 01010001 | 81 | 51 | Q | |
| 01010010 | 82 | 52 | R | |
| 01010011 | 83 | 53 | S | |
| 01010100 | 84 | 54 | T | |
| 01010101 | 85 | 55 | U | |
| 01010110 | 86 | 56 | V | |
| 01010111 | 87 | 57 | W | |
| 01011000 | 88 | 58 | X | |
| 01011001 | 89 | 59 | Y | |
| 01011010 | 90 | 5A | Z | |
| 01011011 | 91 | 5B | [ | |
| 01011100 | 92 | 5C | \ | |
| 01011101 | 93 | 5D | ] | |
| 01011110 | 94 | 5E | ^ | |
| 01011111 | 95 | 5F | _ | |
| 01100000 | 96 | 60 | ` | |
| 01100001 | 97 | 61 | a | |
| 01100010 | 98 | 62 | b | |
| 01100011 | 99 | 63 | c | |
| 01100100 | 100 | 64 | d | |
| 01100101 | 101 | 65 | e | |
| 01100110 | 102 | 66 | f | |
| 01100111 | 103 | 67 | g | |
| 01101000 | 104 | 68 | h | |
| 01101001 | 105 | 69 | i | |
| 01101010 | 106 | 6A | j | |
| 01101011 | 107 | 6B | k | |
| 01101100 | 108 | 6C | l | |
| 01101101 | 109 | 6D | m | |
| 01101110 | 110 | 6E | n | |
| 01101111 | 111 | 6F | o | |
| 01110000 | 112 | 70 | p | |
| 01110001 | 113 | 71 | q | |
| 01110010 | 114 | 72 | r | |
| 01110011 | 115 | 73 | s | |
| 01110100 | 116 | 74 | t | |
| 01110101 | 117 | 75 | u | |
| 01110110 | 118 | 76 | v | |
| 01110111 | 119 | 77 | w | |
| 01111000 | 120 | 78 | x | |
| 01111001 | 121 | 79 | y | |
| 01111010 | 122 | 7A | z | |
| 01111011 | 123 | 7B | { | |
| 01111100 | 124 | 7C | | | |
| 01111101 | 125 | 7D | } | |
| 01111110 | 126 | 7E | ~ | |
| 01111111 | 127 | 7F | DEL (delete) | 删除 |
9 制表符 \t
10 换行符 \r
13 回车符 \n
2 浏览器对url的编码
浏览器会对表单中的key/val 进行编码之后再传递 php脚本会自动将之解码
编码规则： %+字符对应的十六进制
这个是HTML实体的对照表：
php函数中 html_entity_decode 函数将HTML实体转化成对应的char，
HTML实体表示方式 &entity_name; &#numner;（数字可以是10进制与十六进制x两种）
php函数 html_entity_decode 的一个bug就是 不能识别 没有分号的，但是这种没有分号的却可以被浏览器识别
|
字符 |
十进制字符编号 | 实体名字 | 说明 |
| --- | � | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- | 	 | --- | 制表符Horizontal tab |
| --- | | --- | 换行Line feed |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- | | --- | 回车Carriage Return |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
| --- |  | --- | 未使用Unused |
|   | --- | Space | |
| ! | ! | --- | 惊叹号Exclamation mark |
| " | " | " | 双引号Quotation mark |
| # | # | --- | 数字标志Number sign |
| $ | $ | --- | 美元标志Dollar sign |
| % | % | --- | 百分号Percent sign |
| & | & | & | Ampersand |
| " | ' | --- | 单引号Apostrophe |
| ( | ( | --- | 小括号左边部分Left parenthesis |
| ) | ) | --- | 小括号右边部分Right parenthesis |
| * | * | --- | 星号Asterisk |
| + | + | --- | 加号Plus sign |
| , | , | --- | 逗号Comma |
| - | - | --- | 连字号Hyphen |
| . | . | --- | 句号Period (fullstop) |
| / | / | --- | 斜杠Solidus (slash) |
| 0 | 0 | --- | 数字0 Digit 0 |
| 1 | 1 | --- | 数字1 Digit 1 |
| 2 | 2 | --- | 数字2 Digit 2 |
| 3 | 3 | --- | 数字3 Digit 3 |
| 4 | 4 | --- | 数字4 Digit 4 |
| 5 | 5 | --- | 数字5 Digit 5 |
| 6 | 6 | --- | 数字6 Digit 6 |
| 7 | 7 | --- | 数字7 Digit 7 |
| 8 | 8 | --- | 数字8 Digit 8 |
| 9 | 9 | --- | 数字9 Digit 9 |
| : | : | --- | 冒号Colon |
| ; | ; | --- | 分号Semicolon |
| < | < | < | 小于号Less than |
| = | = | --- | 等于符号Equals sign |
| > | > | > | 大于号Greater than |
| ? | ? | --- | 问号Question mark |
| @ | @ | --- | Commercial at |
| A | A | --- | 大写A Capital A |
| B | B | --- | 大写B Capital B |
| C | C | --- | 大写C Capital C |
| D | D | --- | 大写D Capital D |
| E | E | --- | 大写E Capital E |
| F | F | --- | 大写F Capital F |
| G | G | --- | 大写G Capital G |
| H | H | --- | 大写H Capital H |
| I | I | --- | 大写J Capital I |
| J | J | --- | 大写K Capital J |
| K | K | --- | 大写L Capital K |
| L | L | --- | 大写K Capital L |
| M | M | --- | 大写M Capital M |
| N | N | --- | 大写N Capital N |
| O | O | --- | 大写O Capital O |
| P | P | --- | 大写P Capital P |
| Q | Q | --- | 大写Q Capital Q |
| R | R | --- | 大写R Capital R |
| S | S | --- | 大写S Capital S |
| T | T | --- | 大写T Capital T |
| U | U | --- | 大写U Capital U |
| V | V | --- | 大写V Capital V |
| W | W | --- | 大写W Capital W |
| X | X | --- | 大写X Capital X |
| Y | Y | --- | 大写Y Capital Y |
| Z | Z | --- | 大写Z Capital Z |
| [ | [ | --- | 中括号左边部分Left square bracket |
| / | \ | --- | 反斜杠Reverse solidus (backslash) |
| ] | ] | --- | 中括号右边部分Right square bracket |
| ^ | ^ | --- | Caret |
| _ | _ | --- | 下划线Horizontal bar (underscore) |
| ` | ` | --- | 尖重音符Acute accent |
| a | a | --- | 小写a Small a |
| b | b | --- | 小写b Small b |
| c | c | --- | 小写c Small c |
| d | d | --- | 小写d Small d |
| e | e | --- | 小写e Small e |
| f | f | --- | 小写f Small f |
| g | g | --- | 小写g Small g |
| h | h | --- | 小写h Small h |
| i | i | --- | 小写i Small i |
| j | j | --- | 小写j Small j |
| k | k | --- | 小写k Small k |
| l | l | --- | 小写l Small l |
| m | m | --- | 小写m Small m |
| n | n | --- | 小写n Small n |
| o | o | --- | 小写o Small o |
| p | p | --- | 小写p Small p |
| q | q | --- | 小写q Small q |
| r | r | --- | 小写r Small r |
| s | s | --- | 小写s Small s |
| t | t | --- | 小写t Small t |
| u | u | --- | 小写u Small u |
| v | v | --- | 小写v Small v |
| w | w | --- | 小写w Small w |
| x | x | --- | 小写x Small x |
| y | y | --- | 小写y Small y |
| z | z | --- | 小写z Small z |
| ｛ | { | --- | 大括号左边部分Left curly brace |
| | | | | --- | 竖线Vertical bar |
| ｝ | } | --- | 大括号右边部分Right curly brace |
| ~ | ~ | --- | Tilde |
| --- |  | --- | 未使用Unused |
|   | | 空格Nonbreaking space | |
| ? | ¡ | ¡ | Inverted exclamation |
| ￠ | ¢ | ¢ | 货币分标志Cent sign |
| ￡ | £ | £ | 英镑标志Pound sterling |
| ¤ | ¤ | ¤ | 通用货币标志General currency sign |
| ￥ | ¥ | ¥ | 日元标志Yen sign |
| | | ¦ | ¦ or &brkbar; | 断竖线Broken vertical bar |
| § | § | § | 分节号Section sign |
| ¨ | ¨ | ¨ or ¨ | 变音符号Umlaut |
| ? | © | © | 版权标志Copyright |
| a | ª | ª | Feminine ordinal |
| ? | « | « | Left angle quote, guillemet left |
| ? | ¬ | ¬ | Not sign |
| ­ | ­ | Soft hyphen | |
| ? | ® | ® | 注册商标标志Registered trademark |
| ˉ | ¯ | ¯ or &hibar; | 长音符号Macron accent |
| ° | ° | ° | 度数标志Degree sign |
| ± | ± | ± | 加或减Plus or minus |
| 2 | ² | ² | 上标2 Superscript two |
| 3 | ³ | ³ | 上标3 Superscript three |
| ′ | ´ | ´ | 尖重音符Acute accent |
| μ | µ | µ | Micro sign |
| ? | ¶ | ¶ | Paragraph sign |
| · | · | · | Middle dot |
| ? | ¸ | ¸ | Cedilla |
| 1 | ¹ | ¹ | 上标1 Superscript one |
| o | º | º | Masculine ordinal |
| ? | » | » | Right angle quote, guillemet right |
| ? | ¼ | ¼ | 四分之一Fraction one-fourth |
| ? | ½ | ½ | 二分之一Fraction one-half |
| ? | ¾ | ¾ | 四分之三Fraction three-fourths |
| ? | ¿ | ¿ | Inverted question mark |
| à | À | À | Capital A, grave accent |
| á | Á | Á | Capital A, acute accent |
| ? | Â | Â | Capital A, circumflex |
| ? | Ã | Ã | Capital A, tilde |
| ? | Ä | Ä | Capital A, di?esis / umlaut |
| ? | Å | Å | Capital A, ring |
| ? | Æ | Æ | Capital AE ligature |
| ? | Ç | Ç | Capital C, cedilla |
| è | È | È | Capital E, grave accent |
| é | É | É | Capital E, acute accent |
| ê | Ê | Ê | Capital E, circumflex |
| ? | Ë | Ë | Capital E, di?esis / umlaut |
| ì | Ì | Ì | Capital I, grave accent |
| í | Í | Í | Capital I, acute accent |
| ? | Î | Î | Capital I, circumflex |
| ? | Ï | Ï | Capital I, di?esis / umlaut |
| D | Ð | Ð | Capital Eth, Icelandic |
| ? | Ñ | Ñ | Capital N, tilde |
| ò | Ò | Ò | Capital O, grave accent |
| ó | Ó | Ó | Capital O, acute accent |
| ? | Ô | Ô | Capital O, circumflex |
| ? | Õ | Õ | Capital O, tilde |
| ? | Ö | Ö | Capital O, di?esis / umlaut |
| × | × | × | 乘号Multiply sign |
| ? | Ø | Ø | Capital O, slash |
| ù | Ù | Ù | Capital U, grave accent |
| ú | Ú | Ú | Capital U, acute accent |
| ? | Û | Û | Capital U, circumflex |
| ü | Ü | Ü | Capital U, di?esis / umlaut |
| Y | Ý | Ý | Capital Y, acute accent |
| T | Þ | Þ | Capital Thorn, Icelandic |
| ? | ß | ß | Small sharp s, German sz |
| à | à | à | Small a, grave accent |
| á | á | á | Small a, acute accent |
| a | â | â | Small a, circumflex |
| ? | ã | ã | Small a, tilde |
| ? | ä | ä | Small a, di?esis / umlaut |
| ? | å | å | Small a, ring |
| ? | æ | æ | Small ae ligature |
| ? | ç | ç | Small c, cedilla |
| è | è | è | Small e, grave accent |
| é | é | é | Small e, acute accent |
| ê | ê | ê | Small e, circumflex |
| ? | ë | ë | Small e, di?esis / umlaut |
| ì | ì | ì | Small i, grave accent |
| í | í | í | Small i, acute accent |
| ? | î | î | Small i, circumflex |
| ? | ï | ï | Small i, di?esis / umlaut |
| e | ð | ð | Small eth, Icelandic |
| ? | ñ | ñ | Small n, tilde |
| ò | ò | ò | Small o, grave accent |
| ó | ó | ó | Small o, acute accent |
| ? | ô | ô | Small o, circumflex |
| ? | õ | õ | Small o, tilde |
| ? | ö | ö | Small o, di?esis / umlaut |
| ÷ | ÷ | ÷ | 除号Division sign |
| ? | ø | ø | Small o, slash |
| ù | ù | ù | Small u, grave accent |
| ú | ú | ú | Small u, acute accent |
| ? | û | û | Small u, circumflex |
| ü | ü | ü | Small u, di?esis / umlaut |
| y | ý | ý | Small y, acute accent |
| t | þ | þ | Small thorn, Icelandic |
| ? | ÿ | ÿ | Small y, umlaut |