PHP で関数パラメータをカスタマイズする方法

function_name(type_hint $var1, type_hint $var2, ...): return_type

function sum(int $a, int $b): int {
  return $a + $b;
}

function_name(type_hint $var1 = default_value, type_hint $var2 = default_value, ...): return_type

function greet(string $name = "World"): string {
  return "Hello, $name!";
}

function_name(type_hint $var1, type_hint $var2 = default_value, ..., type_hint $varN = default_value[]? = default_value_for_optional_param): return_type

function print_user(string $name, ?string $email = null): void {
  if ($email) {
    echo "$name ($email)";
  } else {
    echo $name;
  }
}

function sum_of_squares(int $a, int $b): int {
  return $a * $a + $b * $b;
}

function sum_of_squares(int $a = 0, int $b = 0): int {
  return $a * $a + $b * $b;
}