Detailed explanation of some usage of PHP Global variable definition_PHP tutorial

PHP’s analysis of global variables is: The role of Global is to define global variables, but this global variable does not apply to the entire website, but to the current page, including all files in include or require.

Example

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代码如下	复制代码
$a = 1; $b = 2; function Sum() { global $a, $b; //在里面声明为全局变量 $b = $a + $b; } Sum(); echo $b; ?>

$a = 1;
$b = 2; function Sum()

{

global $a, $b; //Declared as a global variable inside

$b = $a + $b;

}

Sum();

echo $b;

代码如下	复制代码
function f() { global $a; $a=123; } f(); echo $a; //正确,可以使用

?>

Result: 3
If there is no global variable global, the values ​​​​of $a and $b cannot be obtained within the method, so if you want to use external variables in the method, you need to declare this variable as a global variable first, so that you can use it
Summary: PHP Global variables defined within the function body can be used outside the function body, while global variables defined outside the function body cannot be used inside the function body,
$glpbal $a; $a=123; function f() { echo $a; //Error, }

代码如下	复制代码
function Test_Global() {      Test();  }  include 'B.php';   //将include 从局部Test_Global函数中移出 $a = 0 ; Test_Global(); echo $a; ?> //B.php 文件 function Test() {     global $a;     $a =1; } ?>

Look at the following example

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function f()

代码如下	复制代码
//A.php 文件 include 'B.php'; $a =0; Set_Global($a); echo $a; ?> //B.php 文件 function Set_Global(&$var) {     $var=1; } ?>

{ global $a; $a=123; } f(); echo $a; //Correct, you can use

In user-defined functions, a local function scope will be introduced. Any variables used inside a function will be restricted to the local function scope by default (including variables in files imported by include and require)! Explanation: Test_Global in the A.php file is a defined third-party function. This function uses include to import the global global variable of $a in the B.php file, so $a is limited to the Test_Global local function scope, so B The scope of $a in the .php file is within Test_Global, instead of affecting the entire A.php.... Solution: 1. Break out of local function //A.php file

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function Test_Global() { Test(); } include 'B.php'; //Move include from the local Test_Global function $a = 0 ; Test_Global(); echo $a; ?> //B.php file function Test() { global $a; $a =1; } ?>

2. Excellent accessor

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//A.php file include 'B.php'; $a =0; Set_Global($a); echo $a; ?> //B.php file function Set_Global(&$var) { $var=1; } ?>

Let’s take a look at $GLOBALS[]

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代码如下	复制代码
$var1 = 1;  $var2 = 2;  function test1(){       $GLOBALS['var2'] = &$GLOBALS['var1'];  }  test1();  echo $var2;  //1 $var3 = 1;  $var4 = 2;  function test2(){       global $var3,$var4;       $var4 = &$var3;  }  test2();  echo $var4;

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$var1 = 1;
$var2 = 2;
function test1(){
$GLOBALS['var2'] = &$GLOBALS['var1'];
}
test1();
echo $var2; //1

代码如下	复制代码
$var1 = 1;  function test1(){       unset($GLOBALS['var1']);  }  test1();  var_dump($var1);   //NULL $var2 = 1;  function test2(){      global  $var2;       unset($var2);  }  test2();

$var3 = 1;
$var4 = 2;
function test2(){
Global $var3,$var4;
$var4 = &$var3;
}
test2();
echo $var4;

//2 Why does the print result of $var2 be 1, while the print result of $var4 is 2? In fact, it is because the reference of $var3 points to the reference address of $var4. The actual value of $var4 has not changed. The official explanation is: $GLOBALS['var'] is the external global variable itself, and global $var is the reference or pointer of the same name of the external $var.

Maybe this example is not very clear yet, let me introduce another example:

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$var1 = 1; function test1(){ Unset($GLOBALS['var1']); } test1(); var_dump($var1); //NULL $var2 = 1; function test2(){ Global $var2; Unset($var2); } test2(); echo $var2; //1 The value of $var1 is deleted, but the value of $var2 still exists. This proves that $var2 is just an alias reference, and its value has not been changed in any way. In other words, global $var is actually $var = &$GLOBALS['var'], which is just an alias for calling external variables! http://www.bkjia.com/PHPjc/631248.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/631248.htmlTechArticlephp’s analysis of global variables is: The role of Global is to define global variables, but this global variable does not apply to the entire website, but applies to the current page, including all include or require files...