Home Backend Development PHP Tutorial Detailed explanation of the definition and usage examples of PHP variables

Detailed explanation of the definition and usage examples of PHP variables

Apr 27, 2017 pm 05:15 PM

什么是变量?

变量指的就是在程序执行过程中数值可以变化的量,它与我们所讲的常量是相反的,我们常量是只要被定义了,在程序中就不能被改变。而变量是可以变的。

变量用来做什么?

变量就是用来保存我们值,相当于一个存储数据的容器,例如

x=5;

y=6;

z=x+y,

从上面的表达式 z=x+y,我们能够计算出 z 的值是 11。

在 PHP 中,这三个字母就被称为变量。

变量赋值

变量赋值,是指给一个变量具体的数据值,对于字符串和数字类型的变量,可以通过“=”来实现赋值,像上面一样例子一样,x,y是变量,那么5和6就是变量的值了。

了解了变量的作用和赋值,再看一下变量怎么定义的

变量的定义

PHP中的所有变量都是用“$”开头的

像下面这样

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$a=5;

$b=6

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上面的实例就完成了我们对变量的定义与赋值了。

变量名是大小写敏感的,变量名可以是字母或者下划线开头,后面跟随任意的字母、数字或者下划线,像下面这样的变量命名就是正确的:

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<?php

$thisCup="onik";

$_class=&#39;roof&#39;;

?>

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在看下面不正确的变量命名:

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<?php

$1111_car=11112;

$@spcn=&#39;span&#39;;

?>

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变量名不能以数字开头了再有就是变量名不能以字母和下划线以外的其他字符开头,所以上面定义变量的写法是错误的。

Detailed explanation of the definition and usage examples of PHP variables虽然PHP的变量名可以使用下划线开头,但是我们一般不建议这么使用,因为下划线开头的变量实际上通常是PHP系统自带的变量

除了上面的变量直接赋值外,还有两种方式可以声明变量和赋值,一种是变量间的赋值,什么意思呢?看下面的实例

示例

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<?php

$str1=&#39;spcn&#39;;

$str2=$str1;

$str1=&#39;PHP&#39;;

echo $str2;

?>

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代码运行结果:

Detailed explanation of the definition and usage examples of PHP variables

我们并没有给变量$str2直接赋值,而是将变量$str1赋值给$str2,。这就是我们变量间的赋值。

另一种是引用赋值。从PHP4开始,PHP引入了“引用赋值”的概念,就是用不同的名字访问同一个变量内容。当改变其中一个变量的值,另一个也会发生变化。引用赋值使用“&”来表示引用。

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<?php

 

$i="spcn";                       //声明变量$i

$j=&$i;                          //使用引用赋值,这时$j已经赋值为spcn

$i="hello ,$i";                  //重新给$j赋值

 

echo $j;                         //输出变量$j;

echo "<br/>";

echo $i;                         //输出变量$i;

 

?>

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代码运行结果:

Detailed explanation of the definition and usage examples of PHP variables

Detailed explanation of the definition and usage examples of PHP variables 引用和赋值的区别在于,赋值是将原变量内容复制下来,再用一个新的内存空间来保存,而引用则是给变量的内容在起一个名字。相当于一个别名。就好像一些文学爱好者经常会向报纸,杂志投稿件,但一般都不会真名,而是用笔名,这个笔名就可以看作是一个引用。

了解了变量,下一节,我们了解一下“变量的作用域”。

相关视频教程推荐:《php.cn独孤九贱(4)-php视频教程》PHP变量的创建与命名规则

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