包含PHP标记的PHP字符串将HTML注释放在字符串周围

Question

$string1 = '<?php This is a paragraph ?>';
$string2 = "<?php This is a paragrahp ?>";
echo '<pre>';
print_r($GLOBALS);

当我点击查看源代码时，我会看到：<!--?php This is a paragraph ?-->这两个字符串。为什么会这样？