魔术函数没有起作用

b.php

<?php
// it not ok
class testClassb
{
    private $string = "";

    /**
     * testClass constructor.
     */
    public function __construct($string)
    {
        $this->string = $string;
    }

    //call this?
    public function __toString()
    {
        return $this->string;
    }

    // or call this ?
    public function __invoke()  {
        return $this->__toString();
    }
}


function abc(int $arg)
{
    echo $arg;
}

abc(new testClassb('string'));