Thank you Ajeh for trying to help me but I'm 100% sure there is no space above the opening <?php and after the closing ?>

I removed the product_info.php file and then copy the default file from the includes/languages/english directory to the /includes/languages/english/mytemplate directory. The product page still fine.

At the time I insert this line,

PHP Code:
define('TEXT_XSELL_PRODUCTS', 'Related Products'); 
then the product page turns blank once again.