How PHP strings are defined and the difference between single quotes and double quotes

$str =<<< arbitrary tag

.........................

..............................

any mark;

What is the difference between single and double quotation marks?

' escaped very troublesome to study how to escape out

The "" Number can be escaped

Within "" Can parse variables such as $age = 25; $name = "Kitten $age year old"

' Fast because there is no need to resolve priority use '

How PHP strings are defined and the difference between single quotes and double quotes