A brief introduction to php constants and variables

This article brings you a brief introduction to PHP constants and variables. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Constant

1. A constant, as the name suggests, is a normal quantity
2. A constant is a quantity that remains unchanged during the execution of the script
3. The definition of a constant Using

//定义一个常量
define('NAME','wuhen');
//使用一个常量
echo NAME;//输出结果
wuhenecho "<br>";
//判断常量是否被定义
var_dump(defined(&#39;NAME&#39;));//结果为bool(true)

Variables

1. A variable is a container used to temporarily store values, such as numbers, text characters, or arrays, etc.

2. Definition of variables
In PHP, variables are represented by the dollar sign ($) plus a variable name. There is no need to explicitly declare variables in PHP.

3. Naming rules

• The variable name must start with a letter or an underscore "_"

• The variable name can only contain Letters, numbers, and underscores

• Variable names cannot contain spaces

• PHP is a weak type checking language, so variables do not need to be predefined before use , and there is no need to specify the data type

4. Variable assignment
Assignment by value: Use "=" to directly assign the value of the assignment expression to another variable
Assignment by reference : Assign the reference of the assignment expression memory space to another variable

5. Destruction of the variable
Use the unset() function

6. Determine whether the variable exists
Use isset( )Function

//定义一个变量
$a;
//传值赋值
$a = 5;
//引用赋值
$b = &$a;//把$a的地址传给$b
$b = 6;
echo $a;//结果为6
//引用赋值,改变$b等于是改变$a
//销毁变量
unset($a);
//判断变量是否存在
var_dump(isset($a));//结果为bool(false),说明$a不存在已经被销毁

The scope of variables

1. Local variables
 The scope of a variable declared inside a function is the function in which it is located. It is saved in the stack of memory, so it is very fast

2. Global variables

• Contrary to local variables, global variables can be accessed anywhere in the program

• Variables defined outside all functions have their scope within the entire PHP file

• Global variables are used inside functions, and keywords are added before the variables global declaration or use $GLOBAL[”] to access

//全局变量
$a = 1;function fnc(){
    //引用全局变量
    global $a;    
    echo $a;    
    $a = $a+1;    
    //使用全局变量数组引用
    echo $GLOBALS['a'];
}
fnc();//结果为1 2

3. Static variables

• Static variables are a special kind of local variables, static Variables only exist within the function scope

• They will still exist on the stack after the function ends and will not be destroyed

• Add the key before the variable Word static, the variable becomes a static variable

//静态变量
function fnc(){
    //定义一个静态变量
    static $count = 1;    
    echo $count;    
    $count += 1;
}
fnc();//结果为1
fnc();//结果为2
//说明静态变量$count，没有随着函数的结束而销毁

Variable variable

• Variable variable refers to using the value of a variable as the variable Name

• Variable names can be dynamically named and defined using the

• syntax with two dollar signs $, or wrapped with {} Get up

//可变变量
$a = 'b';
$b = 'abcde';
echo $b;   
echo $$a; //相当于$b
echo ${$a}; //相当于$b
//结果都是 abcde

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