PHPのエラー『Fatal Error: Too Few Arguments to Function』の解決方法

function greet($name) {
    echo "Hello, $name!";
}

greet();

Fatal error: Too few arguments to function greet(), 0 passed and exactly 1 expected

function greet($name) {
    echo "Hello, $name!";
}

greet("Alice"); // 正しく動作する

function greet($name = "Guest") {
    echo "Hello, $name!";
}

greet();       // "Hello, Guest!" と表示
greet("Bob");  // "Hello, Bob!" と表示

function sum(...$numbers) {
    return array_sum($numbers);
}

echo sum(1, 2, 3, 4); // 10
echo sum();           // 0（エラーにならない）

class User {
    public $name;

    public function __construct($name) {
        $this->name = $name;
    }
}

$user = new User(); // エラー発生

$user = new User("Charlie");

class User {
    public $name;

    public function __construct($name = "Guest") {
        $this->name = $name;
    }
}

$user = new User(); // OK

function executeCallback($callback) {
    $callback();
}

executeCallback(function($param) {
    echo "Param: $param";
});

function executeCallback($callback) {
    $callback("Data");
}

executeCallback(function($param) {
    echo "Param: $param";
});

php -v