php魔术方法属性重载方法 - 南十字

php有一类很神奇的方法，这些方法是保留方法，通常不会在外部被显式调用，他们使用双下划线(__)开头，他们被称为魔术方法(Magic Methods)。php官方也不建议定义其他双下划线开头的方法。

这次介绍属性重载方法：get/set/isset/unset

<span style="color: #0000ff;">public</span> void __set ( <span style="color: #0000ff;">string</span> <span style="color: #800080;">$name</span> , <span style="color: #0000ff;">mixed</span> <span style="color: #800080;">$value</span><span style="color: #000000;"> )

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">mixed</span> __get ( <span style="color: #0000ff;">string</span> <span style="color: #800080;">$name</span><span style="color: #000000;"> )

</span><span style="color: #0000ff;">public</span> bool __isset ( <span style="color: #0000ff;">string</span> <span style="color: #800080;">$name</span><span style="color: #000000;"> )

</span><span style="color: #0000ff;">public</span> void __unset ( <span style="color: #0000ff;">string</span> <span style="color: #800080;">$name</span> )

这些方法触发的时机，都是在访问不可访问的属性的时候。

如以下：

<span style="color: #008080;"> 1</span> <span style="color: #000000;">php
</span><span style="color: #008080;"> 2</span> 
<span style="color: #008080;"> 3</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> Cls{
</span><span style="color: #008080;"> 4</span> 
<span style="color: #008080;"> 5</span>     <span style="color: #0000ff;">private</span> <span style="color: #800080;">$a</span> = 0<span style="color: #000000;">;
</span><span style="color: #008080;"> 6</span>     <span style="color: #0000ff;">protected</span> <span style="color: #800080;">$b</span> = 1<span style="color: #000000;">;
</span><span style="color: #008080;"> 7</span>     <span style="color: #0000ff;">public</span> <span style="color: #800080;">$c</span> = 2<span style="color: #000000;">;
</span><span style="color: #008080;"> 8</span>     <span style="color: #0000ff;">var</span> <span style="color: #800080;">$d</span> = 3<span style="color: #000000;">;
</span><span style="color: #008080;"> 9</span> 
<span style="color: #008080;">10</span>     <span style="color: #0000ff;">private</span> <span style="color: #800080;">$_properties</span> = <span style="color: #0000ff;">array</span>(0<span style="color: #000000;">);
</span><span style="color: #008080;">11</span> 
<span style="color: #008080;">12</span>     <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> __set(<span style="color: #800080;">$name</span>, <span style="color: #800080;">$value</span><span style="color: #000000;">){
</span><span style="color: #008080;">13</span>         <span style="color: #0000ff;">echo</span> <span style="color: #ff00ff;">__METHOD__</span> . ' is called! ' . json_encode(<span style="color: #008080;">func_get_args</span>()) . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">14</span>         <span style="color: #0000ff;">return</span> <span style="color: #800080;">$this</span>->_properties[<span style="color: #800080;">$name</span>] = <span style="color: #800080;">$value</span><span style="color: #000000;">;
</span><span style="color: #008080;">15</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">16</span> 
<span style="color: #008080;">17</span>     <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> __get(<span style="color: #800080;">$name</span><span style="color: #000000;">){
</span><span style="color: #008080;">18</span>         <span style="color: #0000ff;">echo</span> <span style="color: #ff00ff;">__METHOD__</span> . ' is called! ' . json_encode(<span style="color: #008080;">func_get_args</span>()) . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">19</span>         <span style="color: #0000ff;">return</span> <span style="color: #800080;">$this</span>->_properties[<span style="color: #800080;">$name</span><span style="color: #000000;">];
</span><span style="color: #008080;">20</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">21</span> 
<span style="color: #008080;">22</span>     <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> __isset(<span style="color: #800080;">$name</span><span style="color: #000000;">){
</span><span style="color: #008080;">23</span>         <span style="color: #0000ff;">echo</span> <span style="color: #ff00ff;">__METHOD__</span> . ' is called! ' . json_encode(<span style="color: #008080;">func_get_args</span>()) . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">24</span>         <span style="color: #0000ff;">return</span> <span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$this</span>->_properties[<span style="color: #800080;">$name</span><span style="color: #000000;">]);
</span><span style="color: #008080;">25</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">26</span> 
<span style="color: #008080;">27</span>     <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span> __unset(<span style="color: #800080;">$name</span><span style="color: #000000;">){
</span><span style="color: #008080;">28</span>         <span style="color: #0000ff;">echo</span> <span style="color: #ff00ff;">__METHOD__</span> . ' is called! ' . json_encode(<span style="color: #008080;">func_get_args</span>()) . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">29</span>         <span style="color: #0000ff;">unset</span>(<span style="color: #800080;">$this</span>->_properties[<span style="color: #800080;">$name</span><span style="color: #000000;">]);
</span><span style="color: #008080;">30</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">31</span> 
<span style="color: #008080;">32</span>     <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">function</span><span style="color: #000000;"> dump(){
</span><span style="color: #008080;">33</span>         <span style="color: #0000ff;">echo</span> "====================== DUMP START ====================\n"<span style="color: #000000;">;
</span><span style="color: #008080;">34</span>         <span style="color: #0000ff;">echo</span> STR
<span style="color: #008080;">35</span>     a: <span style="color: #800080;">$this</span>-><span style="color: #000000;">a
</span><span style="color: #008080;">36</span>     b: <span style="color: #800080;">$this</span>-><span style="color: #000000;">b
</span><span style="color: #008080;">37</span>     c: <span style="color: #800080;">$this</span>-><span style="color: #000000;">c
</span><span style="color: #008080;">38</span>     d: <span style="color: #800080;">$this</span>-><span style="color: #000000;">d\n
</span><span style="color: #008080;">39</span> <span style="color: #000000;">STR;
</span><span style="color: #008080;">40</span>         <span style="color: #0000ff;">foreach</span>(<span style="color: #800080;">$this</span>->_properties <span style="color: #0000ff;">as</span> <span style="color: #800080;">$k</span> => <span style="color: #800080;">$v</span><span style="color: #000000;">){
</span><span style="color: #008080;">41</span>             <span style="color: #0000ff;">echo</span> STR
<span style="color: #008080;">42</span>     Property <span style="color: #800080;">$k</span>: <span style="color: #800080;">$v</span><span style="color: #000000;">\n
</span><span style="color: #008080;">43</span> <span style="color: #000000;">STR;
</span><span style="color: #008080;">44</span> <span style="color: #000000;">        }
</span><span style="color: #008080;">45</span>     
<span style="color: #008080;">46</span>         <span style="color: #0000ff;">echo</span> "====================== DUMP STOP =====================\n"<span style="color: #000000;">;
</span><span style="color: #008080;">47</span>         
<span style="color: #008080;">48</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">49</span> <span style="color: #000000;">}
</span><span style="color: #008080;">50</span> <span style="color: #800080;">$string</span> = 'abcdef'<span style="color: #000000;">;
</span><span style="color: #008080;">51</span> <span style="color: #800080;">$obj</span> = <span style="color: #0000ff;">new</span><span style="color: #000000;"> Cls();
</span><span style="color: #008080;">52</span> 
<span style="color: #008080;">53</span> <span style="color: #800080;">$list</span> = <span style="color: #0000ff;">array</span>('isset', 'get', 'set', 'isset', 'unset', 'isset'<span style="color: #000000;">);
</span><span style="color: #008080;">54</span> <span style="color: #800080;">$obj</span>-><span style="color: #000000;">dump();
</span><span style="color: #008080;">55</span> <span style="color: #0000ff;">foreach</span>(<span style="color: #800080;">$list</span> <span style="color: #0000ff;">as</span> <span style="color: #800080;">$method</span><span style="color: #000000;">){
</span><span style="color: #008080;">56</span>     <span style="color: #0000ff;">for</span>(<span style="color: #800080;">$i</span> = 0 ; <span style="color: #800080;">$i</span> strlen(<span style="color: #800080;">$string</span>) ; <span style="color: #800080;">$i</span> ++<span style="color: #000000;">){
</span><span style="color: #008080;">57</span>         <span style="color: #800080;">$char</span> = <span style="color: #800080;">$string</span>{<span style="color: #800080;">$i</span><span style="color: #000000;">};
</span><span style="color: #008080;">58</span>         <span style="color: #0000ff;">echo</span> "------ <span style="color: #800080;">$method</span> : <span style="color: #800080;">$char</span> ------\n"<span style="color: #000000;">;
</span><span style="color: #008080;">59</span>         <span style="color: #0000ff;">switch</span>(<span style="color: #800080;">$method</span><span style="color: #000000;">){
</span><span style="color: #008080;">60</span>             <span style="color: #0000ff;">case</span> 'isset':
<span style="color: #008080;">61</span>                 <span style="color: #0000ff;">echo</span> json_encode(<span style="color: #800080;">$tmp</span> = <span style="color: #0000ff;">isset</span>(<span style="color: #800080;">$obj</span>-><span style="color: #800080;">$char</span>)) . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">62</span>                 <span style="color: #0000ff;">break</span><span style="color: #000000;">;
</span><span style="color: #008080;">63</span>             <span style="color: #0000ff;">case</span> 'get':
<span style="color: #008080;">64</span>                 <span style="color: #0000ff;">echo</span> json_encode(<span style="color: #800080;">$tmp</span> = <span style="color: #800080;">$obj</span>-><span style="color: #800080;">$char</span>) . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">65</span>                 <span style="color: #0000ff;">break</span><span style="color: #000000;">;
</span><span style="color: #008080;">66</span>             <span style="color: #0000ff;">case</span> 'set':
<span style="color: #008080;">67</span>                 <span style="color: #0000ff;">echo</span> json_encode(<span style="color: #800080;">$tmp</span> = <span style="color: #800080;">$obj</span>-><span style="color: #800080;">$char</span> = <span style="color: #800080;">$char</span> . "-val") . "\n"<span style="color: #000000;">;
</span><span style="color: #008080;">68</span>                 <span style="color: #0000ff;">break</span><span style="color: #000000;">;
</span><span style="color: #008080;">69</span>             <span style="color: #0000ff;">case</span> 'unset':
<span style="color: #008080;">70</span>                 <span style="color: #0000ff;">unset</span>(<span style="color: #800080;">$obj</span>-><span style="color: #800080;">$char</span><span style="color: #000000;">);
</span><span style="color: #008080;">71</span>                 <span style="color: #0000ff;">break</span><span style="color: #000000;">;
</span><span style="color: #008080;">72</span>             <span style="color: #0000ff;">default</span>:
<span style="color: #008080;">73</span>                 <span style="color: #0000ff;">break</span><span style="color: #000000;">;
</span><span style="color: #008080;">74</span> <span style="color: #000000;">        }
</span><span style="color: #008080;">75</span> <span style="color: #000000;">    }
</span><span style="color: #008080;">76</span>     <span style="color: #800080;">$obj</span>-><span style="color: #000000;">dump();
</span><span style="color: #008080;">77</span> }

结果输出：

====================== DUMP START ====================<span style="color: #000000;">
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>: 2<span style="color: #000000;">
    d</span>: 3<span style="color: #000000;">
    Property </span>0: 0
====================== DUMP STOP =====================
------ <span style="color: #0000ff;">isset</span> : a ------<span style="color: #000000;">
Cls</span>::__isset is called! ["a"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : b ------<span style="color: #000000;">
Cls</span>::__isset is called! ["b"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : c ------
<span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : d ------
<span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : e ------<span style="color: #000000;">
Cls</span>::__isset is called! ["e"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : f ------<span style="color: #000000;">
Cls</span>::__isset is called! ["f"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
====================== DUMP START ====================<span style="color: #000000;">
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>: 2<span style="color: #000000;">
    d</span>: 3<span style="color: #000000;">
    Property </span>0: 0
====================== DUMP STOP =====================
------ get : a ------<span style="color: #000000;">
Cls</span>::__get is called! ["a"<span style="color: #000000;">]
</span><span style="color: #0000ff;">null</span>
------ get : b ------<span style="color: #000000;">
Cls</span>::__get is called! ["b"<span style="color: #000000;">]
</span><span style="color: #0000ff;">null</span>
------ get : c ------
2
------ get : d ------
3
------ get : e ------<span style="color: #000000;">
Cls</span>::__get is called! ["e"<span style="color: #000000;">]
</span><span style="color: #0000ff;">null</span>
------ get : f ------<span style="color: #000000;">
Cls</span>::__get is called! ["f"<span style="color: #000000;">]
</span><span style="color: #0000ff;">null</span>
====================== DUMP START ====================<span style="color: #000000;">
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>: 2<span style="color: #000000;">
    d</span>: 3<span style="color: #000000;">
    Property </span>0: 0
====================== DUMP STOP =====================
------ set : a ------<span style="color: #000000;">
Cls</span>::__set is called! ["a","a-val"<span style="color: #000000;">]
</span>"a-val"
------ set : b ------<span style="color: #000000;">
Cls</span>::__set is called! ["b","b-val"<span style="color: #000000;">]
</span>"b-val"
------ set : c ------
"c-val"
------ set : d ------
"d-val"
------ set : e ------<span style="color: #000000;">
Cls</span>::__set is called! ["e","e-val"<span style="color: #000000;">]
</span>"e-val"
------ set : f ------<span style="color: #000000;">
Cls</span>::__set is called! ["f","f-val"<span style="color: #000000;">]
</span>"f-val"
====================== DUMP START ====================<span style="color: #000000;">
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>: c-<span style="color: #000000;">val
    d</span>: d-<span style="color: #000000;">val
    Property </span>0: 0<span style="color: #000000;">
    Property a</span>: a-<span style="color: #000000;">val
    Property b</span>: b-<span style="color: #000000;">val
    Property e</span>: e-<span style="color: #000000;">val
    Property f</span>: f-<span style="color: #000000;">val
</span>====================== DUMP STOP =====================
------ <span style="color: #0000ff;">isset</span> : a ------<span style="color: #000000;">
Cls</span>::__isset is called! ["a"<span style="color: #000000;">]
</span><span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : b ------<span style="color: #000000;">
Cls</span>::__isset is called! ["b"<span style="color: #000000;">]
</span><span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : c ------
<span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : d ------
<span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : e ------<span style="color: #000000;">
Cls</span>::__isset is called! ["e"<span style="color: #000000;">]
</span><span style="color: #0000ff;">true</span>
------ <span style="color: #0000ff;">isset</span> : f ------<span style="color: #000000;">
Cls</span>::__isset is called! ["f"<span style="color: #000000;">]
</span><span style="color: #0000ff;">true</span>
====================== DUMP START ====================<span style="color: #000000;">
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>: c-<span style="color: #000000;">val
    d</span>: d-<span style="color: #000000;">val
    Property </span>0: 0<span style="color: #000000;">
    Property a</span>: a-<span style="color: #000000;">val
    Property b</span>: b-<span style="color: #000000;">val
    Property e</span>: e-<span style="color: #000000;">val
    Property f</span>: f-<span style="color: #000000;">val
</span>====================== DUMP STOP =====================
------ <span style="color: #0000ff;">unset</span> : a ------<span style="color: #000000;">
Cls</span>::__unset is called! ["a"<span style="color: #000000;">]
</span>------ <span style="color: #0000ff;">unset</span> : b ------<span style="color: #000000;">
Cls</span>::__unset is called! ["b"<span style="color: #000000;">]
</span>------ <span style="color: #0000ff;">unset</span> : c ------
------ <span style="color: #0000ff;">unset</span> : d ------
------ <span style="color: #0000ff;">unset</span> : e ------<span style="color: #000000;">
Cls</span>::__unset is called! ["e"<span style="color: #000000;">]
</span>------ <span style="color: #0000ff;">unset</span> : f ------<span style="color: #000000;">
Cls</span>::__unset is called! ["f"<span style="color: #000000;">]
</span>====================== DUMP START ====================<span style="color: #000000;">
Cls</span>::__get is called! ["c"<span style="color: #000000;">]
Cls</span>::__get is called! ["d"<span style="color: #000000;">]
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>:<span style="color: #000000;"> 
    d</span>:<span style="color: #000000;"> 
    Property </span>0: 0
====================== DUMP STOP =====================
------ <span style="color: #0000ff;">isset</span> : a ------<span style="color: #000000;">
Cls</span>::__isset is called! ["a"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : b ------<span style="color: #000000;">
Cls</span>::__isset is called! ["b"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : c ------<span style="color: #000000;">
Cls</span>::__isset is called! ["c"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : d ------<span style="color: #000000;">
Cls</span>::__isset is called! ["d"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : e ------<span style="color: #000000;">
Cls</span>::__isset is called! ["e"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
------ <span style="color: #0000ff;">isset</span> : f ------<span style="color: #000000;">
Cls</span>::__isset is called! ["f"<span style="color: #000000;">]
</span><span style="color: #0000ff;">false</span>
====================== DUMP START ====================<span style="color: #000000;">
Cls</span>::__get is called! ["c"<span style="color: #000000;">]
Cls</span>::__get is called! ["d"<span style="color: #000000;">]
    a</span>: 0<span style="color: #000000;">
    b</span>: 1<span style="color: #000000;">
    c</span>:<span style="color: #000000;"> 
    d</span>:<span style="color: #000000;"> 
    Property </span>0: 0
====================== DUMP STOP =====================

特别强调，当这个属性不存在(基于当前访问权限)的时候，才会触发这些方法。因此严重建议使用确定的键值进行get/set处理，特别是对应的键值不要有访问权限问题，如上例中的a和b，会导致在类内部、类外部、子类内处理的方式不同，开发时产生不可预知的返回值，极易产生bug。

empty方法不能调用属性重载方法。

<span style="color: #800080;">$tmp</span> = <span style="color: #800080;">$obj</span>-><span style="color: #800080;">$char</span> = <span style="color: #800080;">$char</span> . "-val"

此例中，__set自身的返回值将被忽略，同样__get也不会被调用。

此外：属性重载方法必须声明为public，同时参数不能使用引用传递，静态属性不能通过这些方法重载，这些方法也不能被声明为static。

当然，使用魔术方法的时候，也会对反射系统造成影响。