Difference Between Single & Double Quotes in PHP

What’s the difference between single & double quotes in PHP. This question is generally asked in interviews to check how well you understand the concepts. Also if you know the difference it will help you in your day to day programming.

In PHP you can specify  the strings using four different ways single quotes , double quotes, heredoc and newdoc syntax.  But most of the time single & double quotes is used to specify strings.