Question

$obj = new stdClass();
echo gettype($obj); //object

function abc(object $obj) {
    return;
}

abc($obj); //Catchable fatal error: Argument 1 passed to abc() must be an instance of object, instance of stdClass given

Why calling abc($obj) triggers error?

Catchable fatal error: Argument 1 passed to abc() must be an instance of object, instance of stdClass given

Answer 1

Since PHP 5 (Jul 2004; Zend Engine 2) type hinting worked only for class and interface names, array (PHP 5.1; Nov 2005) or callable (PHP 5.4; Mar 2012). There was no common ancestor object (like in some other programming languages like C#) in PHP object/type model. What you had to specify was stdClass

function abc(stdClass $obj) {
    return;
}

As of PHP 7.2 (Nov 2017) it’s possible to use object type hint exactly as you guessed in your question:

function abc(object $obj) {
    return;
}

Cf. Type declarations in the PHP manual.