PHP abstract class – Stack Overflow in Spanish

Hi, I have to do a business class, but using the abstract method, because I have read so little I have understood that it is handled more so, but there must be some error in my code because I get a blank screen: /

    owner = $ owner;
    $ this-> purpose = $ purpose;
}

abstract public function set Owner ($ owner);

abstract public function setFinality ($ purpose);

}

/ **
* Extended class for private company
 * /
 class EmpresaPrivada extends Company
{
public function setFinality ()
{
    $ this-> purpose = "lucrative";
    echo "The company has a purpose:". $ this-> purpose;
}
public function set Owner ($ owner)
{
    $ this-> owner = "private";
    echo "The company has an owner:". $ this-> owner;
}

}

/ **
* Extended class for public company
* /
class CompanyPublica extends Company
{
public function setFinality ()
{
    $ this-> purpose = "help";
    echo "The company has a purpose:". $ this-> purpose;
}
public function set Owner ($ owner)
{
    $ this-> owner = "public";
    echo "The company has an owner:". $ this-> owner;
}
}

/ **
 * Extended class for joint venture
 * /
 class EmpresaMixta extends Company
{
public function setFinality ()
{
    $ this-> purpose = "both";
    echo "The company has a purpose:". $ this-> purpose;
}
public function set Owner ($ owner)
{
    $ this-> owner = "mixed";
    echo "The company has an owner:". $ this-> owner;
}
 }

/ **
 * Extended class for company self-management
* /
class CompanyAutogestion extends Company
{
public function setFinality ()
{
    $ this-> purpose = "support";
    echo "The company has a purpose:". $ this-> purpose;
}
public function set Owner ($ owner)
{
    $ this-> owner = "own";
    echo "The company has an owner:". $ this-> owner;
}

}

$ obj = new Private Company ();
$ obj-> setFinality ();

or in any case, what is the correct way to use this type of functions, thank you very much