When it comes to web development there are so many resources out there on the internet that you can use for reference, ask for help or to find out how a new technology or programming language works. I am only going to talk about the 5 websites I use the most to help me with development of websites.
Stackoverflow was created in 2008 by Jeff Atwood and Joel Spolsky. It is a question and answer website which is used by a lot of developer worldwide. What makes Stackoverflow so great it that you can find an answer to almost any question that you might have. There is a great chance that someone came across the problem you are facing a long time ago and someone already helped him/her. Since Stackoverflow is an open question and answer website, it is not required of you to have an account in order to see the questions and answers.
W3Schools is a website that is used to learn web technologies online. I usually use this website for mostly checking up on simple functions in PHP and JavaScript. What makes W3Schools so great is that they allow you to try out or play around with the code on their website using their online editor, and execute the code in a so called sanbox. Through that you can see how the code is supposed to be implemented and what result it will give you.
PHP.net is the website of the programming language PHP. I use PHP.net a lot to check up on PHP functions and how to use them. This is also what makes PHP.net so great, every single PHP function that exists is listed on this website from which to which version of PHP these functions are supported.
So why did I put YouTube on this list. It is quite simple actually. YouTube is a really great video search engine. Since I am not the best person when it comes to reading how a web technology works I find it a lot easier when I can watch someone else do it and then just following them like that. On YouTube can find almost any kind of video and the library of development videos is amazing. You could learn all the basics of web development by simply learning it from YouTube.
I left Google for last as this one could be debated about. As Google is a search engine and because how great their search is, it sometimes happen if I can't find an answer on any of the other websites Google is the next place I go to. It helps me find websites that could have the answer I am looking for but never heard of.