How can the use of global variables in PHP scripts impact functionality and best practices, particularly when migrating to newer PHP versions?

Using global variables in PHP scripts can impact functionality by making code harder to maintain, debug, and test. It can lead to unexpected behavior and make it difficult to track where variables are being modified. When migrating to newer PHP versions, global variables may not be supported or may behave differently, causing compatibility issues. To solve this issue, it is best to avoid using global variables and instead use function parameters or class properties to pass data between different parts of the code.

// Avoid using global variables
$globalVar = "Hello";

function exampleFunction() {
    global $globalVar;
    echo $globalVar;
}

exampleFunction();
```

Instead, consider passing variables as parameters:

```php
// Pass variables as parameters
$globalVar = "Hello";

function exampleFunction($localVar) {
    echo $localVar;
}

exampleFunction($globalVar);