What is the difference between global variables and local variables in php

Difference: 1. Local variables are variables defined inside the function and can only be used within the function in which they are defined; global variables are variables defined outside all functions, and their scope is any part of the current source code. place, but is not available inside the function. 2. Local variables will be automatically destroyed when the function call ends, and global variables will not be destroyed until the program ends.

The operating environment of this tutorial: windows7 system, PHP7.1 version, DELL G3 computer

The scope of variables (the range that can be used ) is called variable scope. Variables must be used within their valid scope. If they exceed the valid scope, the variable will lose its meaning. PHP variables can be divided into two types according to scope: global variables and local variables.

Variables that can be used anywhere in the current source code (outside the function) are called global variables, and they have a global scope; variables that can only be used inside the function in which they are defined are called local variables. Has local scope.

In PHP, global variables cannot be used directly inside the function; and local variables cannot be used directly outside the function. Of course, this is not absolute. Through some methods, it is still possible to call global variables within a function and call local variables outside the function. We will introduce these in later studies.

Local variables and local scope

Local variables are variables defined inside a function. They can only be defined within the function in which they are defined. use. Local variables are automatically destroyed at the end of the function call.

[Example] The following defines a function named example, and defines a local variable a inside the function, and then tries to output the value of variable a inside the function and outside the function. The specific code is as follows:

<?php
header("Content-type:text/html;charset=utf-8");
function example() {
	$a = "欢迎来到PHP中文网！";
	// 在函数内定义变量 a
	echo "局部变量 a 的值为：" . $a . "<br>";
}

example();
if ($a) {// 在函数外部调用 $a,如果 $a 存在则会打印下面的内容
	echo "在函数外部调用函数内的局部变量 a，其值为：" . $a;
}
?>

The running results are as follows:

It can be seen from the running results that the local variables defined inside the function cannot be called outside the function, because the local variable $a The scope is the example() function in which it is defined and cannot be used outside the function.

Global variables and global scope

Global variables are variables defined outside all functions, and their scope is anywhere in the current source code , but it is not available inside the function. Global variables will always exist while the program is running and will be destroyed only when the program ends.

[Example] Define a global variable a, and output the value of the global variable inside and outside the function. The specific code is as follows:

<?php
header("Content-type:text/html;charset=utf-8");
$a = "欢迎来到PHP中文网！";
// 在函数外部定义全局变量 a
function example() {
	if ($a) {
		echo "在函数内部调用全局变量 a，其值为：" . $a;
	}
}

example();
echo "在函数外部调用全局变量 a，其值为：" . $a;
?>

The running results are as follows:

It can be seen from the running results that $a is not successfully called inside the function, but it can be called outside the function.

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