Echo"hello"; and echo($sum); here in this two codes their is a point that echo must have single quotes r double quotes

Question

but in echo($sum); their is no single r double quotes why

Answer 1

php, echo compiles the variables in double quotes but does not so for variables in single codes. try changing the echo from below code to double quotes or vice versa, you would notice the difference.
https://code.sololearn.com/w3IXipJlUULM/?ref=app