Detailed explanation of the role and usage of the global keyword in PHP

Detailed explanation of the role and usage of the global keyword in PHP

In PHP programming, we often encounter situations where we need to access global variables within a function. PHP provides a global keyword to solve this problem. In this article, we will introduce in detail the role and use of the global keyword.

The global keyword is used to declare local variables as global variables, so that the value of the global variable can be accessed and modified within the function. After declaring a variable using the global keyword inside a function, the scope of the variable will be extended to the entire script file.

Using the global keyword is very simple. You only need to use the global keyword inside the function to declare the global variables that need to be accessed. After the declaration, you can use and modify the value of the global variable just like accessing ordinary variables. Here is an example:

$x = 10;

function myFunction() {
  global $x;
  echo $x;
}

myFunction();

In the above example, we have declared a global variable $x and declared it as a global variable using the global keyword inside the function. Then the value of the global variable $x can be directly accessed and output inside the function.

It should be noted that if the value of a global variable is modified inside the function, it will also be affected outside the function. For example, modify the code in the above example as follows:

$x = 10;

function myFunction() {
  global $x;
  $x = 20;
}

myFunction();
echo $x;

In the above example, we modify the value of global variable $x to 20 inside the function, and then output the value of global variable $x outside the function. The output result will be 20, indicating that modifying the value of the global variable inside the function can affect the outside of the function.

In addition to using the global keyword, PHP also provides the $GLOBALS[] array to access global variables. The keys of this array are the names of the global variables and the values ​​are the values ​​of the global variables. Here is an example:

$x = 10;

function myFunction() {
  $GLOBALS['x'] = 30;
}

myFunction();
echo $x;

In the above example, we use the $GLOBALS[] array inside the function to access the global variable $x and modify its value to 30. Then output the value of the global variable $x outside the function. The output result will be 30, indicating that the value of global variables can also be accessed and modified through the $GLOBALS[] array.

It should be noted that when using the global keyword or the $GLOBALS[] array to access global variables, the name of the variable is in string form and needs to be enclosed in quotes.

During the programming process, we may sometimes encounter situations where we need to access the same global variable in multiple files. In this case, we can use PHP’s superglobal variable $_GLOBALS. This variable is an associative array that stores all global variables. Here is an example:

$x = 10;

function myFunction() {
  $_GLOBALS['x'] = 40;
}

myFunction();
echo $x;

In the above example, we use the $_GLOBALS[] array inside the function to access the global variable $x and modify its value to 40. Then output the value of the global variable $x outside the function. The output result will be 40, indicating that the value of global variables can also be accessed and modified through the $_GLOBALS[] array.

To sum up, the global keyword is the keyword used to access global variables in PHP. Use the global keyword to declare local variables as global variables, allowing you to access and modify the value of the global variable within the function. In addition to the global keyword, you can also use the $GLOBALS[] array and the $_GLOBALS superglobal variable to access and modify global variables. For large PHP projects, reasonable use of the global keyword and global variables can conveniently manage and operate global data.

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