First, phpMyAdmin is a PHP script for creating, modify, and deleting users, tables, etc. in MySQL. It is not able to read from, or write to, the serial port. Nor does it provide a means of accessing specific data from a specific table via a web browser (it simply uses a web browser as the display mechanism).
You need a PHP script that is able to access the MySQL database that contains the data of interest and is able to perform the SELECT statement needed to get the data of interest. That PHP script needs to talk to the serial port or it needs to be on a web server that the Arduino as client can query or it needs to make a GET request of the Arduino as server.
You need to define exactly how the Arduino is to be connected to the PC that PHP is running on.