About why php echo can output objects

I saw a sample code of a PayPal document before, and I added echo directly in front of the class object, and actually printed a bunch of strings.

It turns out that __toString can be processed. When an object is treated as a string, this method will be called.

class A{

    function __toString(){
        return "我输出了字符串";
    }

}

$a=new A();
echo $a;