Why are global variables invalid or wrong?

Before this article introduces the invalid problem of global variables in PHP, let me briefly introduce to you, what is the PHP global variable global? First of all, the phpglobal scope does not apply to the entire website, but to the current page, including all files in include or require. However, global variables defined within the function body can be used within the function body, while global variables defined outside the function body cannot be used within the function body.

I also mention here that the php $GLOBALS variable refers to all variables available in the global scope. In the $GLOBALS array, each variable is an element, the key name corresponds to the variable name, and the value corresponds to the variable's content. . $GLOBALS exists in the global scope because $GLOBALS is a superglobal variable. Pay attention to the writing method of $GLOBALS. For example, the variable $a1 is written as $GLOBALS['a1'].

Then let’s use a simple and easy-to-understand example to explain in detail the problem that global is invalid in php:

First demo1.php page code As follows:

<?php
    $site_name = &#39;CodePlayer&#39;;
    function sayHi(){
        global $site_name;
        echo "Hello! Welcome to $site_name !";
    }
    ?>

The demo2.php page code is as follows:

<?php
    function include_view_page(){
        include &#39;demo2.php&#39;;
        sayHi();                    
    }
    include_view_page();
    ?>

When we visit the demo1.php page, the welcome statement can be displayed correctly. However, when we use the browser to access the demo2.php page, the output result is as follows:

Hello! Welcome to !

When we call the sayHi() function in the function include_view_page(), the global $site_name in the sayHi() function of the demo2.php page But it was not correctly recognized and took effect.

In fact, when we include in the function include_view_page() When using the demo2.php page, the variable $site_name of the demo2.php page is equivalent to being stored in the scope within the include_view_page() function. Everyone knows that global a variable within a function actually establishes a reference to the global variable of the page within the function. In this example, the $site_name variable is only a local variable within the include_view_page() function for demo1.php, so the global variable cannot be displayed. When we make related calls, we cannot obtain the correct variables and variable values.

In PHP, we especially need to pay attention to the above-mentioned problem of including a page within a function, causing the scope of the variables in the page to change. In order to avoid this situation, we should try to reduce multi-level include calls and try not to use include within functions. In addition, we can also declare $site_name as a global variable in the demo2.php page.

//demo2.php
    <?php
    global $site_name;
    $site_name = &#39;CodePlayer&#39;;
    function sayHi(){
        global $site_name;
        echo "Hello! Welcome to $site_name !";
    }
    ?>

The example of how to reference global variables in a function is as follows:

The output result is as follows:

var2 is set to ' xxxxxx'
var2 is set to 'eeee'

is visible, in the above code:
$var2 =&$var1; //1 is only visible inside the function. And $GLOBALS["var2"] =&$var1; //2 is visible in the global scope.

I hope this article will help everyone about the invalid problem of php global!

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