What is different between using " " and ' ' in php

Question

using " and '

Answer 1

variables will be replaced with their values between ", but will not between '.
$a = 5;
echo "$a\n";
echo '$a\n';

Answer 2

there are two types of strings. First is here doc and second one is quote string. In quote string there are two types of method- ", ' single quote and double quote. In single quote all things written on it will be used. And on double quote it will look for variable first before using strings anywhere.