Php Basics-difference between double quotes and single quotes

Php Basics-differences between double quotation marks and single quotation marks in php's previous cognition, I think there is no difference between single quotation marks and double quotation marks. if single quotation marks are used outside, double quotation marks are used inside, if double quotation marks are used outside, single quotation marks are used in them. it is always thought that single quotation marks and double quotation marks are the difference, but there is still a difference.

The difference is that the content in the double quotation mark string can be interpreted and replaced, while the content in the single quotation mark string is always considered as a common character.

 
The result will be output: hello world! $ Str