conditional extension

Bar.php

class Bar extends DsFieldBase {
    public function __construct($foo) {
      // do some other stuff...
    }
}

Foo.php

class Foo extends DsFieldBase {
    public function __construct($foo) {
      // do some stuff...
    }
}

Meh.php

class Foo extends DsFieldBase {
    public function __construct($foo) {
      if ($condition) {
        return new Foo($foo);
      }
      else {
        return new Bar($foo);
      }
    }
}