If you don't add any quotation marks after echo, you can also display text information. Why do you need to add single quotation marks? Is there any difference?

Question

If you don’t add any quotation marks after echo, text information can also be displayed. Why do you need to add single quotation marks? Is there any difference?

Answer 1

Single quotes support escaping but do not support output variables. According to the code format specifications, not adding quotes is not recognized. It will appear to others that you are not rigorous enough.

Answer 2

Single quotes: plain text. Double quotes: supports escaping such as \\ and outputs \. {$a} outputs the contents of variable $a=xxx.

Answer 3

If there happens to be a string that is a constant, it is another matter