<?hh

<<__EntryPoint>>
function main(): void {
    $counter = 0;
    $isDark = false;

    function updateUI(): void {
        global $counter, $isDark;
        echo "Counter: $counter\n";
        echo "Theme: " . ($isDark ? "Dark" : "Light") . "\n";
    }

    function increment(): void {
        global $counter;
        $counter += 1;
        updateUI();
    }

    function decrement(): void {
        global $counter;
        $counter -= 1;
        updateUI();
    }

    function reset(): void {
        global $counter;
        $counter = 0;
        updateUI();
    }

    function toggleTheme(): void {
        global $isDark;
        $isDark = !$isDark;
        updateUI();
    }

    // Simulate actions
    updateUI();
    increment();
    increment();
    toggleTheme();
    decrement();
    reset();
}

Demonstrates a simple counter with theme toggling using Hack variables, functions, and conditional statements.

<?hh
function fib(int $n): int {
    return $n < 2 ? $n : fib($n-1) + fib($n-2);
}
<<__EntryPoint>>
function main(): void {
    for ($i = 0; $i < 10; $i++) {
        echo fib($i) . "\n";
    }
}

Generates first 10 Fibonacci numbers using recursion.

<?hh
function factorial(int $n): int {
    return $n === 0 ? 1 : $n * factorial($n - 1);
}
<<__EntryPoint>>
function main(): void {
    echo factorial(5) . "\n";
}

Calculates factorial of a number using recursion.

<?hh
function isPrime(int $n): bool {
    if ($n < 2) return false;
    for ($i = 2; $i < $n; $i++) {
        if ($n % $i === 0) return false;
    }
    return true;
}
<<__EntryPoint>>
function main(): void {
    echo isPrime(13) ? "Prime\n" : "Not Prime\n";
}

Checks if a number is prime.

<?hh
<<__EntryPoint>>
function main(): void {
    $nums = vec[1,2,3,4,5];
    $sum = array_sum($nums);
    echo $sum . "\n";
}

Calculates sum of an array of numbers.

<?hh
<<__EntryPoint>>
function main(): void {
    $s = "HELLO";
    echo strrev($s) . "\n";
}

Reverses a string.

<?hh
<<__EntryPoint>>
function main(): void {
    $n = 5;
    for ($i = 1; $i <= 10; $i++) {
        echo "$n x $i = " . ($n * $i) . "\n";
    }
}

Prints multiplication table of a number.

<?hh
<<__EntryPoint>>
function main(): void {
    $c = 25;
    $f = ($c * 9 / 5) + 32;
    echo $f . "\n";
}

Converts Celsius to Fahrenheit.

<?hh
<<__EntryPoint>>
function main(): void {
    $temp = 80;
    $thresh = 75;
    echo ($temp > $thresh ? "Alarm: Temperature Too High!\n" : "Temperature Normal\n");
}

Simulates an alarm if a threshold is exceeded.

<?hh
<<__EntryPoint>>
function main(): void {
    $steps = 10;
    $pos = 0;
    for ($i = 0; $i < $steps; $i++) {
        $pos += (rand(0,1) === 0 ? -1 : 1);
        echo $pos . "\n";
    }
}

Simulates a 1D random walk.