Using double quotes means you can use variables without having to concatenate it on separately.
So, this would work:
echo "Hello $username!";
But this wouldn't:
echo 'Hello $username!';
If you use single quotes and you want to use variables as well, you need to do something like this:
echo 'Hello ' . $username . '!';
//or, slightly faster:
echo 'Hello ', $username, '!';
Using single quotes is a bit faster than double quotes because PHP doesn't need to look through the quoted strings to see if there are any variables.