PHP warnings and notices do not directly affect Google's SEO ranking of a website. However, they can indicate issues with the website's code, which can lead to other problems that could impact SEO, such as slow page load times, broken links, and missing content. Additionally, having a large number of PHP warnings or notices could suggest that the website is poorly maintained, which can affect user experience and negatively impact the website's reputation, which can in turn affect SEO.
It's important to address PHP warnings and notices, not only for SEO purposes but also for the stability and reliability of the website. Making sure that your website is free of PHP warnings and notices can help ensure that it runs smoothly and provides a positive user experience, which is important for both visitors and search engines.