Description:
------------
There is not possible to define return type parent object in abstract method,

Online demo: https://3v4l.org/innXW

From documentation:
The same types are available for return type declarations as are available for argument type declarations[1]

The parameter must be an ==INSTANCEOF== the given class or interface name[2]

[1] http://php.net/manual/en/functions.returning-values.php#functions.returning-values.type-declaration

[2] http://php.net/manual/en/functions.arguments.php#functions.arguments.type-declaration.types

Test script:
---------------
<?php

abstract class Entity {}
class FooEntity extends Entity {}

abstract class Repository
{
    abstract public function getDetail(): Entity;
}

class FooRepository extends Repository
{
    public function getDetail(): FooEntity
    {
        return new FooEntity;
    }
    
}

$obj = new FooRepository();
$obj->getDetail();

Expected result:
----------------
No error expected.

Actual result:
--------------
Fatal error: Declaration of FooRepository::getDetail(): FooEntity must be compatible with Repository::getDetail(): Entity in /in/innXW on line 18