Getting the following : Notice: Use of undefined constant php – assumed 'php'

I am trying to look for a solution to this error.

Notice:
Use of undefined constant php – assumed ‘php’ in /hermes/walnaweb09a/b1694/pow.mitrasen/htdocs/wp-content/themes/peace_tree/content-page.php on line 12

<?php
/**
 * The template used for displaying page content
 *
 * @package WordPress
 * @subpackage Twenty_Fifteen
 * @since Twenty Fifteen 1.0
 */
?>

        <?php // Post thumbnail. //twentyfifteen_post_thumbnail(); ?>
        <?php// the_title(); ?>
        <?php // the_content(); ?>
        <?php echo get_the_excerpt(); ?>

Is this because of trailing characters on content-page ? If not what else could be the reason. Or could it be because of a plugin. Would like some help. Thanks.