Write 'According to Wikipedia' in a PhD thesis

"According to some unknown guy on the Internet, I can say that according to Wikipedia..."

No, you should avoid that. Wikipedia is open, so it can be changed by (almost) everyone and you have no guarantee that the given information there is correct. While many articles are very informative, it might be better to take the actual source (given on the Wikipedia page) and make sure that this source is credible (e.g. published in a good, peer reviewed journal).

I think that Dirk’s answer is highly misleading and represents a popular, but false view of the website. Investigations have shown that Wikipedia contains as many or fewer statements of incorrect fact on scientific and technical matters as more traditional encyclopedias like Encyclopedia Britannica (though "mistakes by omission" are more common on Wikipedia). Just because Wikipedia can be edited by anyone doesn’t mean it’s unreliable. Additionally, just because anyone can add content doesn’t mean that there are no controls on content, nor does it mean that there aren’t people or systems for double checking edits.

However, it is correct that one shouldn’t cite Wikipedia because Wikipedia (like most encyclopedias) is a tertiary source and has a strict “no original research” policy. Encyclopedias are summaries of information, but don't contain original research by design and are not the original source of the content. You should always cite the original source of the content, not a reproduction of it. If you wish to credit an encyclopedia with drawing your attention to the material, you can append "accessed via Wikipedia" to a citation, which I've seen done in some published scholarly books and journal articles.

A good Wikipedia article must be supported with credible sources and references (see here and here). Wikipedia itself has a good read on citing Wikipedia.

In case of scientific articles, the references are usually either published books and/or articles and papers from scientific journals and conferences. In either case, you should be able to follow them and reference the Wikipedia article (or the section of your choice) back to any of these original sources, and then cite that original source in your thesis. That makes for a valid and accepted citation instead of something that someone might have randomly written on the day you checked the article.