Example analysis of variables and constants in PHP

PHP has four different variable scopes: static, parameter, global, local.

1. Global variables

are in All variables defined outside the function have global scope, and variables declared inside the function are local variables and can only be accessed within the function. To use global variables in a function, use the global keyword.

 <?php
$a = 1;
$b = 2;
function Sum()
{
    global $a, $b;
    $b = $a + $b;
}
Sum();
?>

The second way to access variables in the global scope is to use a special PHP custom $GLOBALS array. The previous example can be written as:

 <?php
$a = 1;
$b = 2;
function Sum()
{
    $GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"];
}
Sum();
?>

2. Static variable

Another important feature of variable scope is static variable (staticvariable). Static variables only exist in the local function scope, but their values ​​are not lost when program execution leaves this scope.

Static variables defined in a function cannot be called outside the function.

Static variables also provide a way to deal with recursive functions. A recursive function is a function that calls itself. Be careful when writing recursive functions, as they may recurse indefinitely. You must ensure that there are adequate means to terminate recursion.

 <?php
function Test()
{
    static $count = 0;
    $count++;
    echo $count;
    if ($count < 10) {
        Test ();
    }
    $count--;
}
?>

3. Local variables

Parameters are local variables that pass values ​​to the function through the calling code.

4. Variable variables

Sometimes it is very convenient to use variable variable names. That is, the variable name of a variable can be set and used dynamically. An ordinary variable is set by declaration, for example:

## A mutable variable obtains the value of an ordinary variable as the variable name of the mutable variable. In the above example,

<span style="color:#000000;">##e6f80b953bad177c6e29aaa5fa5de61f<span style="color:#0000BB;"></span></span>

hello

uses two dollar signs ($), and it can be used as a variable variable. For example:

# #At this time, both variables are defined: the content of $a is "hello" and the content of

##73bdf98888a97702c045f179b68360f3

$hello

is "world". Therefore, it can be expressed as:

##757e33a8cd50cd90edf94bd11f060aa4The following way of writing is more accurate And the same result will be output:

##d09b4fd39db8af2073c162d786b9b151

它们都会输出：hello world。

要将可变变量用于数组，必须解决一个模棱两可的问题。这就是当写下$$a[1]时，解析器需要知道是想要$a[1]作为一个变量呢，还是想要$$a作为一个变量并取出该变量中索引为 [1]的值。解决此问题的语法是，对第一种情况用${$a[1]}，对第二种情况用${$a}[1]。

注意可变变量不能用于 PHP 的超全局变量数组。这意味着不能这样用：${$_GET}。

5. 常量

<?php
define("GREETING", "Welcome to W3School.com.cn!");
echo GREETING;
?>

• 常量前面没有美元符号（$）；

• 常量默认为大小写敏感。按照惯例常量标识符总是大写的。

• 常量只能用 define() 函数定义，而不能通过赋值语句；

• 和 superglobals 一样，常量的范围是全局的。不用管作用域就可以在脚本的任何地方定义和访问常量；

• 常量一旦定义就不能被重新定义或者取消定义；

• 常量的值只能是标量数据（boolean，integer，float 和 string）或 null。

1. 自定义常量

<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?>

2. 类常量

    可以在类中定义常量，常量的值必须是一个定值，不能是变量，类属性或其它操作（如函数调用）的结果。但在PHP5.6中，对常量进行了增强，允许常量计算，允许使用包    含数字、字符串字面值和常量的表达式结果来定义const常量。常量的值也可以为一个数组，但不能是变量。

    定义类常量只能使用const关键字。

    class MyClass {
        const AB = 2;
        public function showConstant(){
            echo self::AB;
        }
    }

 const 与 define 的区别？

1、const用于类成员变量的定义，一经定义，不可修改。Define不可以用于类成员变量的定义，可用于全局常量。

2、Const可在类中使用，define不能

3、Const不能再条件语句中定义常量

4、const采用普通的常量名称，define可以采用表达式作为名称

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