In most countries, the PhD supervisor is one of the examiners of their student’s doctoral thesis. They are equally responsible for evaluating the thesis and providing a judgement. In many countries, doctoral theses need to be reviewed by additional examiners that might be native or foreign. The thesis examination is also usually followed by a PhD viva or defense presentation. PhD supervisors commonly serve as examiners for this as well.
This practice is followed in most countries for doctoral theses evaluations.