echo function does not work in server side php script when using php 8.1

Question

When upgrading to php 8.1, the echo function does not work in server side script, such as:

<?
  echo $data;
?>

but it works on html file for display.

I already have the following configuration in the apache httpd conf file:

LoadModule php_module "C:/PHP/php8apache2_4.dll"