PHP uses global to define a solution to global variables that has no effect

The

variables defined by global are global variables. For example, if the variables in function and the variables with the same name outside the function are not the same variable in php, if we use global If the same variable name is defined in the function, then they are a variable. Let's take a few examples to illustrate.

For PHP beginners, when using the global keyword, you may find that a variable outside a function is globalized within a function, but in some cases the variable cannot be output correctly (that is, the global variable is invalid ). Let's look at a simple and common example.

Here, we have two pages a.php and b.php.

b.php page code is as follows:

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

a.php page code is as follows:

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

The above example is very simple. We hope that when we visit the a.php page, The welcome message can be displayed correctly. However, unfortunately, when we use the browser to access the a.php page, we find that the output is as follows:

Hello! Welcome to !

In other words, when we call the sayHi() function in the function include_view_page(), b.php The global $site_name in the page sayHi() function is not correctly recognized and effective. What exactly is this matter about?

In fact, when we include the b.php page in the function include_view_page(), the variable $site_name of the b.php page is equivalent to being stored in the scope within the include_view_page() function. As we all know, globalizing a variable within a function actually establishes a reference to the global variable of the page within the function. In our example, the $site_name variable is only a local variable within the include_view_page() function for a.php, so the variable cannot be globalized. Naturally, we cannot obtain the correct variables and variable values ​​when we make related calls. .

In PHP, we especially need to pay attention to the above-mentioned problem of including a page within a function, causing the scope of 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 in the form of a global variable in the b.php page. The code is as follows:

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

For example, to reference global variables within a function, first look at the following code:

<?php  
$var1 = "#####";  
$var2 = "&&&&&"; 
function global_references($use_globals)  
{  
global $var1, $var2;  
if (!$use_globals) {  
$var2 =&$var1; //1 
} else {  
$GLOBALS["var2"] =&$var1; //2 
}  
} 
global_references(false);  
echo "var2 is set to &#39;$var2&#39;<br/>";  
global_references(true);  
echo "var2 is set to &#39;$var2&#39;<br/>";  
?>

The output result is as follows:

var2 is set to '&&&&&' 
var2 is set to '#####'

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.

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