PHP中變量的作用域主要有三種：全局、局部和靜態(tài)。 1. 全局作用域是指在函數(shù)外部聲明的變量，只能在函數(shù)外部訪問(wèn)，若需在函數(shù)內(nèi)部使用，必須用global關(guān)鍵字引入；2. 局部作用域是指在函數(shù)內(nèi)部聲明的變量，僅在該函數(shù)內(nèi)部有效，函數(shù)執(zhí)行完畢後變量被銷毀；3. 靜態(tài)作用域通過(guò)static關(guān)鍵字實(shí)現(xiàn)，使局部變量在多次函數(shù)調(diào)用間保留其值，但不改變其局部訪問(wèn)特性。

When working with PHP, you'll run into different variable scopes —which basically means where a variable is accessible in your code. The main ones you need to know are global , local , and static . Let's break them down so you can understand when and why each one matters.

Global Scope: Variables Outside Functions

Variables declared outside of any function are in the global scope. That means they're accessible from anywhere in your script except inside functions, unless you specifically tell PHP to use them there.

For example:

 $message = "Hello, world!";

function sayHello() {
    echo $message; // This won't work as expected
}

sayHello(); // Outputs nothing or throws an error

To access a global variable inside a function, you have to use the global keyword:

 $message = "Hello, world!";

function sayHello() {
    global $message;
    echo $message;
}

sayHello(); // Outputs: Hello, world!

This tells PHP that inside this function, we want to use the $message variable from the global scope.

Local Scope: Variables Inside Functions

Variables declared inside a function are only available within that function. They're said to have local scope. Trying to access them outside will result in an error or undefined behavior.

Example:

 function sayHi() {
    $greeting = "Hi there!";
    echo $greeting;
}

sayHi(); // Outputs: Hi there!
echo $greeting; // Error: Undefined variable

Once the function finishes running, the variable $greeting is destroyed (unless it's declared as static, which we'll get to next).

Static Sc??ope: Preserving Local Variable Values

The static keyword lets a local variable keep its value between function calls. Normally, when a function ends, all its variables are deleted. But if a variable is declared static , it retains its value for the next time the function runs.

Useful for things like counters:

 function countVisits() {
    static $count = 0;
    $count ;
    echo "Visit number: $count <br>";
}

countVisits(); // Visit number: 1
countVisits(); // Visit number: 2
countVisits(); // Visit number: 3

Without static , $count would reset to 0 every time, and you'd always see "Visit number: 1".

A few things to note:

• You can only declare a variable as static inside a function.
• It doesn't make the variable globally accessible—it just keeps it around longer than usual.

So in short:

• Use global when you really need to bring a variable into a function from the outside.
• Stick to local variables whenever possible—they're easier to manage and less likely to cause conflicts.
• Reach for static when you want a local variable to remember its state across multiple function calls.

基本上就這些。

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