How can PHP developers handle variable scope and existence in files effectively?

PHP developers can handle variable scope and existence in files effectively by using global and superglobal variables to access variables declared outside of a function's scope, and by checking for variable existence using functions like isset() or empty(). It's also important to properly include files where variables are declared to ensure they are accessible throughout the application.

// Example of handling variable scope and existence in PHP files

// Declaring a global variable
$globalVar = "I am a global variable";

function testFunction() {
    // Accessing a global variable inside a function
    global $globalVar;
    echo $globalVar;
    
    // Checking if a variable exists before using it
    if(isset($localVar)) {
        echo $localVar;
    } else {
        echo "Local variable does not exist";
    }
}

testFunction();