How to dynamically set the name of a variable in PHP
在上一篇文章中我们简单了解了什么是变量,变量的声明,介绍了变量赋值的两种方式,有需要的可以点击链接查看→《你知道php变量有哪些设置值的方式吗》。这次我们继续来聊聊变量,介绍一下如果动态的设置(改变)一个变量的名称。
我们一般操作变量,都是改变变量的值(这是所有的语言都支持的),有时会改变一下变量的类型;但很少(几乎没有)进行改变变量名称的操作的(支持这一操作的语言也很少)。但在PHP中可以,它提供了一个特殊的其他类型的变量——可变变量,可以允许我们动态的设置(修改)一个变量的名称。
我们来看看下面这个示例:
<?php $str='trans'; //声明变量$str $trans='hello!'; //声明变量$trans echo $str; //输出变量$str echo "<br/>"; echo $$str; //通过可变变量输出$trans的值 ?>
可以看出,我们定义两个变量$str
和$trans
,首先输出变量$str
的值,然后使用可变变量来改变变量$str
的名称,输出改变名称后的变量值。看看输出结果:
trans hello!
那么具体什么是可变变量呢?
所谓“可变变量”,就是一个变量以另外一个变量的值作为变量名。利用这一特性我们可以动态的设置和使用一个变量的变量名。例如,我们可以设置$tireqty
的值:
$varname = 'tireqty';
于是,我们就可以用varname
取代$tireqty
。例如,我们可以设置$tireqty
的值:
varname = 5;
这个代码等价于:
$tireqty = 5;
而声明可变变量的方法非常简单,只需要在变量名前加上至少两个“$
”符号即可。
<?php $demo = 'string'; $$demo = 'hello!'; echo $string; ?>
输出结果:
hello!
利用可变变量可以动态设置变量名称的特性,我们可以将可变变量应用在表单处理中。
在使用表单获取数据时,往往不会只有一个数据
<form action="test.php" method="post"> <table border="0"> <tr bgcolor="#cccccc"> <td width="150">Item</td> <td width="15">Quantity</td> </tr> <tr> <td>Tires</td> <td align="center"><input type="text" name="tireqty" size="3" maxlength="3" /></td> </tr> <tr> <td>Oil</td> <td align="center"><input type="text" name="oilqty" size="3" maxlength="3" /></td> </tr> <tr> <td>Spark Plugs</td> <td align="center"><input type="text" name="sparkqty" size="3" maxlength="3" /></td> </tr> <tr> <td colspan="2" align="center"><input type="submit" value="Submit Order" /></td> </tr> </table> </form>
因此处理表单数据时,也不会只有一个表单变量。而传统的中规中矩的写法是下面这样的,表单变量越多,赋值语句将会很长。
<?php $oilqty = $_POST['oilqty']; $tireqty = $_POST['tireqty']; $sparkqty = $_POST['sparkqty']; echo "<p>Your order is as follows:</p>"; echo "<br />\$oilqty=".$oilqty; echo "<br />\$tireqty=".$tireqty; echo "<br />\$sparkqty=".$sparkqty; ?>
我们利用可变变量来优化,使用一个循环语句和一个变量来自动处理它们
<?php foreach ($_POST as $key => $value) { $$key = $value; } echo "<p>Your order is as follows:</p>"; echo "<br />\$oilqty=".$oilqty; echo "<br />\$tireqty=".$tireqty; echo "<br />\$sparkqty=".$sparkqty; ?>
代码是不是简洁了不少!
可变变量也可以用于数组。
<?php $demo = 'hello'; $test = 'world'; $arr = array('demo', 'test'); echo ${$arr[0]}.'<br>'.${$arr[1]}.'<br>'; ?>
输出结果:
hello world
分析:${$arr[0]}
和${$arr[1]}
为什么中间要用“{ }
”将数组变量包裹住呢?
因为,如果你写下 $$a[1]
时,解析器需要知道你是想要 $a[1]
作为一个变量呢,还是想要把$$a
当成变量、[1]
指的是这个变量的索引。
解决这个问题的语法是使用“{ }”将变量包裹,对应的语法是:第一种情况使用${$a[1]}
,第二种情况使用${$a}[1]
。
好了就说到这里了,有其他想知道的,可以点击这个哦。→ →php视频教程
最后给大家推荐一个PHP数组的免费视频教程:从0开始进入PHP的世界,快来学习吧!
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