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PHP学习笔记--有关php中的变量_PHP教程

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Jul 20, 2016 am 11:13 AM

php 变量可变学习手册笔记

PHP中的可变变量(更多请自行参考php手册):

　　php中的可变变量说白了，就是拿一个变量的值解析成一个变量名，去读那个变量名的值。实例：

<?<span php
    </span><span $a</span> = "China";  <span //</span><span 变量a</span>
    <span $b</span> = "a";       <span //</span><span 变量b</span>
    <span $China</span> = "I'm Chinese !"; <span //</span><span 变量China</span>
    <span $f</span> = "b";  <span //</span><span 变量f</span>
    
    <span echo</span> <span $a</span>."<br />";  <span //</span><span 输出 China</span>
    <span echo</span> $<span $a</span>."<br />"; <span //</span><span 输出 I'm Chinese  --这里像要当做可变变量解析，必须在前面多加一个$符号</span>
    <span $a</span> = "f";  <span //</span><span 改变变量指向的名称(这里就是可变变量的应用)</span>
    <span echo</span> $<span $a</span>."<br />"; <span //</span><span 经过上面指向变量f后输出 b</span>
    <span $a</span> = "b"; <span //</span><span 同上</span>
    <span echo</span> $<span $a</span>."<br /><br />"; <span //</span><span 输出 a</span>
    
    <span echo</span> <span $b</span>."<br />"; <span //</span><span 输出 a</span>
    <span echo</span> $<span $b</span>."<br />"; <span //</span><span 输出 b</span>
    <span echo</span> $$<span $b</span>."<br /><br />"; <span //</span><span 输出 a</span>
    
    <span echo</span> <span $f</span>."<br />"; <span //</span><span 输出 b</span>
    <span echo</span> $<span $f</span>."<br />"; <span //</span><span 输出 a</span>
    <span echo</span> $$<span $f</span>."<br />"; <span //</span><span 输出 b</span>
    <span echo</span> $$$<span $f</span>."<br /><br />"; <span //</span><span 输出 a</span>
<span     
    $</span><span $a</span> = "China"; <span //</span><span 前面最后一个已经更改了变量为b所谓$$a=$b 也就是改变的$b的值</span>
    <span echo</span> <span $b</span>."<br />"; <span //</span><span 输出 China</span>
    <span echo</span> $<span $b</span>; <span //</span><span 输出 I'm Chinese</span>
?>

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注意：可变变量不能应用于$this和超全局变量 (php变量的作用域和其他高级编程语言有所不同。看代码)

<?<span php 
    </span><span $name</span> = 'man'<span ; 
    $</span><span $name</span> = 'abc'; <span //</span><span 如果事先没有man这个变量。就新建一个man变量。 然后把abc赋值过去</span>
    $$<span $name</span> = 'def'<span ;
    </span><span echo</span> <span $man</span>."<br />"; <span //</span><span 输出abc</span>
    <span echo</span> <span $abc</span>; <span //</span><span 输出def</span>
    
    <span echo</span> "<br /> <hr />"<span ;
    </span><span function</span><span  show()
    {
        </span><span global</span> <span $name</span>; <span //</span><span 这里的global并不是设置为全局变量。而是引用</span>
        <span echo</span> <span $name</span>."<br />";  <span //</span><span 输出man</span>
<span     }
    
    </span><span function</span><span  showtwo()
    {
        </span><span //</span><span global $name;
        //echo $name."<br />";</span>
        <span echo</span> <span $GLOBALS</span>['name']; <span //</span><span 超全局变量数组</span>
<span     }
    
    show(); 
    showtwo();
</span>?>

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变量函数:

<?<span php
        </span><span function</span><span  b()
        {
            </span><span echo</span> "这是B"<span ;    
        }
        </span><span function</span> c(<span $name</span> = "China") <span //</span><span 设默认值</span>
<span          {
            </span><span echo</span> "这是<span $name</span>"<span ;
        }
        
        </span><span $a</span> = 'b'<span ;
        </span><span $a</span>(); <span //</span><span 找值所在的函数</span>
         <span $a</span> = 'c'<span ;
        </span><span $a</span><span ();<br /> </span>?>

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可变变量的一个典型应用:

<!DOCTYPE html <span PUBLIC</span> "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
</head>

<body>
    
    <div>
        <form action="#" method="post">
            <label>name: </label>
            <input type="text" name="name" /><br />
            <label>pwd : </label>
            <input type="text" name="pwd" /><br />
            <label>tag : </label>
            <input type="text" name="tag" /><br />
            <input type="submit" value="提交" />
        </form>
    </div>
    <?<span php
    
        </span><span foreach</span>(<span $_POST</span> <span as</span> <span $key</span>=><span $value</span><span )
        {        
            </span><span //</span><span print_r($_POST);</span>
            $<span $key</span> = <span $value</span><span ;
        }
        </span><span //</span><span extract($_POST); //从数组中将变量导入到当前的符号表 --自行查找php手册</span>
        <span echo</span> <span $name</span>."<br />"<span ;
        </span><span echo</span> <span $pwd</span>."<br />"<span ;
        </span><span echo</span> <span $tag</span>."<br />"<span ;
    </span>?>
</body>
</html>

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　　补充：

　　变量的特性。如果一个变量事先没有声明，那么如果你要对一个变量赋值，此时PHP在后台的一个操作就是，当你给这个未声明的变量进行赋值时，后台就已经帮你申明了这个变量了。直接看例子：

<?<span php
</span><span class</span><span  A
{
    </span><span public</span> <span function</span><span  show()
    {
        </span><span //</span><span 记住这里的name 实现是没有声明的。</span>
        <span echo</span> (<span isset</span>(<span $this</span>->name)?"true":"false")." -- "<span ; 
        </span><span echo</span> <span $this</span>-><span name;
    }
}

</span><span $A</span> = <span new</span> A(); <span //</span><span 实例化
//直接输出，是没有任何结果的. 因为没有这个变量 。。这里可以用isset判断为false</span>
<span $A</span>->show();  <span //</span><span 输出 "fase -- "

//这里进行赋值,在赋值时，后台默认声明此变量</span>
<span $A</span>->name = "我有输出了。这个变量被声明了!"<span ; 
</span><span echo</span> "<br />"<span ;
</span><span $A</span>->show(); <span //</span><span 输出 "true -- 我有输出了。这个变量被声明了!"</span>
?>

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小结：看了上面的示例后，你们以后再看到别人封装的代码里，如果有那种没有声明变量就直接用的，不要感到奇怪。那是叫你自己去设置呢。你直接赋值用就行了。。。（PS：其实本人一开始也感到迷糊，因为以前在搞.NET的在C#里是绝对不会允许这种情况发生的。强型语言用惯了。。。在看这种弱类型的语言，确实一开始不会很适应）

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