How do features like constants, variable variables, and function calls via variables impact the feasibility of modifying PHP's language structure?

Features like constants, variable variables, and function calls via variables in PHP can make modifying the language structure more feasible because they allow for dynamic and flexible coding. Constants provide a way to define fixed values that cannot be changed, variable variables allow for dynamic variable names, and function calls via variables enable the execution of functions based on runtime values. These features make it easier to adapt and extend PHP's language structure without having to make extensive changes to the code.

// Example of using constants, variable variables, and function calls via variables
define('MY_CONSTANT', 'Hello World');

$variableName = 'MY_CONSTANT';
echo $$variableName; // Output: Hello World

function myFunction() {
    return 'Dynamic Function Call';
}

$functionName = 'myFunction';
echo $functionName(); // Output: Dynamic Function Call