Jeez damn curly quotes! Above copy and paste got converted in pasting and made the top working example slanting/curly too, had to paste and edit to get the correct straight apostrophes. If you cannot see the difference Control + a few times to zoom in.
The first code at top of this page has slanting ones all through, hence the problems.
Works – <?php echo ‘<p>Hello World</p>’; ?>
Doesn’t work – <?php echo ‘<p>Hello World</p>’; ?>