Refer: https://www.hostinger.com/tutorials/wordpress/how-to-add-php-code-to-wordpress-post-or-page
The plug-in is called "Insert PHP" and this version worked, as of "2017-10-16;
Upgrading to PHP 5+ from PHP 4+
The special directives file name has changed from php.ini to php5.ini. If you need special directives, place them there for now on.
PHP to create PDF
Print PDF's without installing any special plug-ins for php. Necessary, because most hosting ISP's do not install PDF support by default.
http://www.fpdf.org/