Variable Scope

In this lesson, we're going to talk about something called 'scope'. Scope refers to the visibility of variables. In other words, where can we access a variable from?

Global Scope

A variable that is defined outside of a function is said to be in the global scope. let's define a variable in the global scope:

$name = 'Alice';

Let's see what happens when we try to access this variable from inside of a function:

function sayHello() {
    echo 'Hello ' . $name; // Error
}

If you run this code, you will get an error. If you come from a JavaScript background, you may be surprised by this. JavaScript has lexical scoping, which means that functions can access variables from their containing scope automatically. PHP does not have lexical scoping. We have to explicitly tell the function that we want to use the $name variable from the global scope.

We can do this by using the global keyword:

function sayHello() {
    global $name;
    echo 'Hello ' . $name;
}

Now you should see the output Hello Alice.

If I try and change the value of the $name variable inside of the function, it will change it in the local scope of the function, but it will not change the value of the $name variable in the global scope:

function sayHello() {
    global $name;
    $name = 'Bob';
    echo 'Hello ' . $name;
}

sayHello(); // Hello Bob

echo $name; // Alice

Local Scope

A variable that is defined inside of a function is said to be in the local scope. Let's define a variable called $names inside of a function and echo out the second name in the array:

function sayGoodbye() {
  $names = ['John', 'Brad', 'Alice'];
  echo 'Goodbye ' . $names[1];
}

sayGoodbye(); // Goodbye Brad

If we try to access the $names variable from outside of the function, we will get an error:

function sayHello() {
    $name = 'Alice';
    echo 'Hello ' . $name;
}

sayHello(); // Hello Alice

echo $names[1]; // Error

We get the error because the $names variable is in the local scope of the function. It is not accessible outside of the function.