Difference between single quote and double quote in PHP

Single quote(' ') and double quote (" ")  both specify strings in PHP. But there some difference.
Single quote(' ')  will not  interpret more escape sequences for special characters:
Example

<?php
$mystring='test';
echo '\r\n';
echo '$mystring';

?>

will output

 \r\n$mystring

 Single quote(" ")  will  interpret more escape sequences for special characters:
Example:

 <?php
$mystring='test';
echo "\r\n";
echo "$mystring"

?>

will output

test

 echo "\r\n";  interpret as return key.