A comment in PHP code is a line that is not executed as a part of the program. Its only purpose is to be read by someone who is looking at the code.
<?php
// This is a single-line comment
# This is also a single-line comment
/* This is a
multi-line comment */
? >
<!DOCTYPE html>
<html>
<body>
<?php
# This is also a single-line comment
echo
"Comments";
?>
</body>
</html>