The difference and use of const and static in php

This article introduces to you the difference and use of const and static in php. Friends in need can refer to it

First of all, about const, members can only be modified inside a php class. Attributes cannot modify methods, as follows:

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class Test{  
    const PATH = 'c/';//修饰常量  
    const function test(){//这种方法是错误的，const无法修饰方法  
    }  
}

const is in use There is no need to add permission modified fields, but it requires skills when calling const modified constants, because the constants belong to the entire class, not to a certain object, so you need to use the class name plus the domain name modifier when calling. Example:

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echo Test::PATH;  
//也可以使用下面的方法调用  
$test = new Test();  
echo  $test::PATH;

But there will be problems when calling inside the class , how to get the current class name or current object name inside the class (php can get the class name based on the object - reflection), php provides us with two special keywords, such as:

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class Test{  
    const PATH = 'c/';//修饰常量  
    public function ass(){  
        echo $this::PATH.'
';  
        echo self::PATH.'
';  
    }  
}

where self (without $) represents the current class name, and $this represents the current object. In this way, we can call const modified constants inside the class.

Next let’s talk about static

#Although const is very useful, the value cannot be modified once it is defined, but sometimes we You need an attribute that belongs to the class, but its value can be modified. For example, if you count how many objects of this class have been constructed, it is time to use static. When static modifies the member variables of the class, its usage method is basically the same as const. The same, the difference is that permissions can be added to statically modified attributes (const can also be added in 7.1 PHP, but not in the past), and the values ​​of statically modified member variables can be modified.

But when the member method is statically modified, the usage method changes a little. Inside the class, inside the statically modified method body, any ordinary member variables of the current class cannot be accessed. To be precise Generally speaking, the keyword $this cannot be used, and only static member variables and class constants of the current class can be used.

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class Test{  
    static public $name='asd';  
    const π = 3.1415926;  
    public $age = 10;  
    public static function lenth($r){  
        //echo $this->age;//会发生错误，不能访问普通的成员变量  
        echo Test::$name;  
        return $r*2*self::π;  
          
    }  
    public static function area($r){  
        return $r*$r*self::π;  
    }     
}

In the process of using static, the method called is the same as calling static The modified variables are different and can be called through class names or object names, for example:

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echo '周长：'. Test::lenth(1).'
';  
echo '面积：'.Test::area(1).'
';  
$per = new Test();  
echo $per->lenth(3);

The above two calling methods are legal.

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