When to use single quotes, double quotes, and backticks in MySQL

I am trying to learn the best way to write queries. I also understand the importance of being consistent. Until now, I have randomly used single quotes, double quotes, and backticks without any real thought. Example: ```php $query = 'INSERT INTO table (id, col1, col2) VALUES (NULL, val1, val2)'; ``` Also, in the above example, consider that `table`, `col1`, `val1`, etc. may be variables. What is the standard for this? What do you do?