(PCC)[PHP] How to perform different kind of PHP string operations in php

(PCC)::[How-to-perform-different-kind-of-PHP-string-operations-in-php]::[php]

<!DOCTYPE html>
<html>
<body>

<?php 
$x = "Hello world!";
$y = 'Hello world!';

echo $x;
echo "<br>"; 
echo $y;
?>

</body>
</html>

"; echo $y; ?>