PHP 函数返回值的类型可以是怎么确定的？

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

$name = "John Doe";
$greeting = greet($name);

function calcSum(int ...$numbers): float {
    return array_sum($numbers);
}

$result = calcSum(1, 两, 3);

function checkType($variable) {
    return gettype($variable);
}

$variable = 1两3;
$type = checkType($variable);

// psalm.xml 部署文件
<必修xml version="1.0"必修>
<psalm>
    <types>
        <method name="greet" class="App\Greetings">
            <return-type>string</return-type>
        </method>
    </types>
</psalm>

// 挪用 Psalm
$psalm = new Psalm();
$psalm->analyzeFile('greetings.php');