在PHP语言中使用JSON_PHP教程
目前，JSON已经成为最流行的数据交换格式之一，各大网站的API几乎都支持它。
我写过一篇《数据类型和JSON格式》，探讨它的设计思想。今天，我想总结一下PHP语言对它的支持，这是开发互联网应用程序（特别是编写API）必须了解的知识。
从5.2版本开始，PHP原生提供json_encode()和json_decode()函数，前者用于编码，后者用于解码。
一、json_encode()
该函数主要用来将数组和对象，转换为json格式。先看一个数组转换的例子：
<ol class="dp-c"> <li class="alt"><span><span class="vars">$arr</span><span> = </span><span class="keyword">array</span><span> (</span><span class="string">'a'</span><span>=>1,</span><span class="string">'b'</span><span>=>2,</span><span class="string">'c'</span><span>=>3,</span><span class="string">'d'</span><span>=>4,</span><span class="string">'e'</span><span>=>5); </span></span></li> <li> <span class="func">echo</span><span> json_encode(</span><span class="vars">$arr</span><span>); </span> </li> </ol>
结果为
<ol class="dp-c"><li class="alt"><span><span>{</span><span class="string">"a"</span><span>:1,</span><span class="string">"b"</span><span>:2,</span><span class="string">"c"</span><span>:3,</span><span class="string">"d"</span><span>:4,</span><span class="string">"e"</span><span>:5} </span></span></li></ol>再看一个对象转换的例子：
<ol class="dp-c"> <li class="alt"><span><span class="vars">$obj</span><span>->body = </span><span class="string">'another post'</span><span>; </span></span></li> <li> <span class="vars">$obj</span><span>->id = 21; </span> </li> <li class="alt"> <span class="vars">$obj</span><span>->approved = true; </span> </li> <li> <span class="vars">$obj</span><span>->favorite_count = 1; </span> </li> <li class="alt"> <span class="vars">$obj</span><span>->status = NULL; </span> </li> <li> <span class="func">echo</span><span> json_encode(</span><span class="vars">$obj</span><span>); </span> </li> </ol>
结果为
<ol class="dp-c">
<li class="alt"><span><span>{ </span></span></li>
<li>
<span class="string">"body"</span><span>:</span><span class="string">"another post"</span><span>, </span>
</li>
<li class="alt">
<span class="string">"id"</span><span>:21, </span>
</li>
<li>
<span class="string">"approved"</span><span>:true, </span>
</li>
<li class="alt">
<span class="string">"favorite_count"</span><span>:1, </span>
</li>
<li>
<span class="string">"status"</span><span>:null </span>
</li>
<li class="alt"><span>} </span></li>
</ol>由于json只接受utf-8编码的字符，所以json_encode()的参数必须是utf-8编码，否则会得到空字符或者null。当中文使用GB2312编码，或者外文使用ISO-8859-1编码的时候，这一点要特别注意。
二、索引数组和关联数组
PHP支持两种数组，一种是只保存"值"（value）的索引数组（indexed array），另一种是保存"名值对"（name/value）的关联数组（associative array）。
由于javascript不支持关联数组，所以json_encode()只将索引数组（indexed array）转为数组格式，而将关联数组（associative array）转为对象格式。
比如，现在有一个索引数组
<ol class="dp-c"> <li class="alt"><span><span class="vars">$arr</span><span> = Array(</span><span class="string">'one'</span><span>, </span><span class="string">'two'</span><span>, </span><span class="string">'three'</span><span>); </span></span></li> <li> <span class="func">echo</span><span> json_encode(</span><span class="vars">$arr</span><span>); </span> </li> </ol>
结果为：
<ol class="dp-c"><li class="alt"><span><span>[</span><span class="string">"one"</span><span>,</span><span class="string">"two"</span><span>,</span><span class="string">"three"</span><span>] </span></span></li></ol>
如果将它改为关联数组：
<ol class="dp-c"> <li class="alt"><span><span class="vars">$arr</span><span> = Array(</span><span class="string">'1'</span><span>=></span><span class="string">'one'</span><span>, </span><span class="string">'2'</span><span>=></span><span class="string">'two'</span><span>, </span><span class="string">'3'</span><span>=></span><span class="string">'three'</span><span>); </span></span></li> <li> <span class="func">echo</span><span> json_encode(</span><span class="vars">$arr</span><span>); </span> </li> </ol>
结果就变了：
<ol class="dp-c"><li class="alt"><span><span>{</span><span class="string">"1"</span><span>:</span><span class="string">"one"</span><span>,</span><span class="string">"2"</span><span>:</span><span class="string">"two"</span><span>,</span><span class="string">"3"</span><span>:</span><span class="string">"three"</span><span>} </span></span></li></ol>注意，数据格式从"[]"（数组）变成了"{}"（对象）。
如果你需要将"索引数组"强制转化成"对象"，可以这样写
<ol class="dp-c"><li class="alt"><span><span>json_encode( (object)</span><span class="vars">$arr</span><span> ); </span></span></li></ol>
或者
<ol class="dp-c"><li class="alt"><span><span>json_encode ( </span><span class="vars">$arr</span><span>, JSON_FORCE_OBJECT ); </span></span></li></ol>
三、类（class）的转换
下面是一个PHP的类：
<ol class="dp-c">
<li class="alt"><span><span class="keyword">class</span><span> Foo { </span></span></li>
<li>
<span class="keyword">const</span><span> ERROR_CODE = </span><span class="string">'404'</span><span>; </span>
</li>
<li class="alt">
<span class="keyword">public</span><span> </span><span class="vars">$public_ex</span><span> = </span><span class="string">'this is public'</span><span>; </span>
</li>
<li>
<span class="keyword">private</span><span> </span><span class="vars">$private_ex</span><span> = </span><span class="string">'this is private!'</span><span>; </span>
</li>
<li class="alt">
<span class="keyword">protected</span><span> </span><span class="vars">$protected_ex</span><span> = </span><span class="string">'this should be protected'</span><span>; </span>
</li>
<li>
<span class="keyword">public</span><span> </span><span class="keyword">function</span><span> getErrorCode() { </span>
</li>
<li class="alt">
<span class="keyword">return</span><span> self::ERROR_CODE; </span>
</li>
<li><span>} </span></li>
<li class="alt"><span>} </span></li>
</ol>现在，对这个类的实例进行json转换：
<ol class="dp-c"> <li class="alt"><span><span class="vars">$foo</span><span> = </span><span class="keyword">new</span><span> Foo; </span></span></li> <li> <span class="vars">$foo_json</span><span> = json_encode(</span><span class="vars">$foo</span><span>); </span> </li> <li class="alt"> <span class="func">echo</span><span> </span><span class="vars">$foo_json</span><span>; </span> </li> </ol>
输出结果是
<ol class="dp-c"><li class="alt"><span><span>{</span><span class="string">"public_ex"</span><span>:</span><span class="string">"this is public"</span><span>} </span></span></li></ol>可以看到，除了公开变量（public），其他东西（常量、私有变量、方法等等）都遗失了。
四、json_decode()
该函数用于将json文本转换为相应的PHP数据结构。下面是一个例子：
<ol class="dp-c">
<li class="alt"><span><span class="vars">$json</span><span> = </span><span class="string">'{"foo": 12345}'</span><span>; </span></span></li>
<li>
<span class="vars">$obj</span><span> = json_decode(</span><span class="vars">$json</span><span>); </span>
</li>
<li class="alt">
<span>print </span><span class="vars">$obj</span><span>->{</span><span class="string">'foo'</span><span>}; </span><span class="comment">// 12345</span><span> </span>
</li>
</ol>通常情况下，json_decode()总是返回一个PHP对象，而不是数组。比如：
<ol class="dp-c">
<li class="alt"><span><span class="vars">$json</span><span> = </span><span class="string">'{"a":1,"b":2,"c":3,"d":4,"e":5}'</span><span>; </span></span></li>
<li>
<span>var_dump(json_decode(</span><span class="vars">$json</span><span>)); </span>
</li>
</ol>结果就是生成一个PHP对象：
<ol class="dp-c">
<li class="alt"><span><span>object(stdClass)#1 (5) { </span></span></li>
<li>
<span>[</span><span class="string">"a"</span><span>] => int(1) </span>
</li>
<li class="alt">
<span>[</span><span class="string">"b"</span><span>] => int(2) </span>
</li>
<li>
<span>[</span><span class="string">"c"</span><span>] => int(3) </span>
</li>
<li class="alt">
<span>[</span><span class="string">"d"</span><span>] => int(4) </span>
</li>
<li>
<span>[</span><span class="string">"e"</span><span>] => int(5) </span>
</li>
<li class="alt"><span>} </span></li>
</ol>如果想要强制生成PHP关联数组，json_decode()需要加一个参数true：
<ol class="dp-c">
<li class="alt"><span><span class="vars">$json</span><span> = </span><span class="string">'{"a":1,"b":2,"c":3,"d":4,"e":5}'</span><span>; </span></span></li>
<li>
<span>var_dump(json_decode(</span><span class="vars">$json</span><span>),true); </span>
</li>
</ol>结果就生成了一个关联数组：
<ol class="dp-c">
<li class="alt"><span><span class="keyword">array</span><span>(5) { </span></span></li>
<li>
<span>[</span><span class="string">"a"</span><span>] => int(1) </span>
</li>
<li class="alt">
<span>[</span><span class="string">"b"</span><span>] => int(2) </span>
</li>
<li>
<span>[</span><span class="string">"c"</span><span>] => int(3) </span>
</li>
<li class="alt">
<span>[</span><span class="string">"d"</span><span>] => int(4) </span>
</li>
<li>
<span>[</span><span class="string">"e"</span><span>] => int(5) </span>
</li>
<li class="alt"><span>} </span></li>
</ol>五、json_decode()的常见错误
下面三种json写法都是错的，你能看出错在哪里吗？
<ol class="dp-c">
<li class="alt"><span><span class="vars">$bad_json</span><span> = </span><span class="string">"{ 'bar': 'baz' }"</span><span>; </span></span></li>
<li>
<span class="vars">$bad_json</span><span> = </span><span class="string">'{ bar: "baz" }'</span><span>; </span>
</li>
<li class="alt">
<span class="vars">$bad_json</span><span> = </span><span class="string">'{ "bar": "baz", }'</span><span>; </span>
</li>
</ol>对这三个字符串执行json_decode()都将返回null，并且报错。
第一个的错误是，json的分隔符（delimiter）只允许使用双引号，不能使用单引号。第二个的错误是，json名值对的"名"（冒号左边的部分），任何情况下都必须使用双引号。第三个的错误是，最后一个值之后不能添加逗号（trailing comma）。
另外，json只能用来表示对象（object）和数组（array），如果对一个字符串或数值使用json_decode()，将会返回null。
<ol class="dp-c"><li class="alt"><span><span>var_dump(json_decode(</span><span class="string">"Hello World"</span><span>)); </span><span class="comment">//null</span><span> </span></span></li></ol>
六、参考材料
* PHP Manual （http://php.net/manual/en/book.json.php）
* Ed Finkler, JSON is Everybody's Friend （http://phpadvent.org/2008/json-is-everybodys-friend-by-ed-finkler）
原文地址：http://www.ruanyifeng.com/blog/2011/01/json_in_php.html
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