PHP/My SQL Database -> to Arduino Serial Monitor
Firstly if you really are using an Uno then you cant read the data without putting the uno on a network (obviously).  To do this you could get either a Ethernet shield of a Wifi shield.  A wifi shield is probably based on an ESP8266 and you talk to it by creating a serial connection (using a software serial port).  There are plenty of examples and messages on here to get you started.

Once that's working you need to write a web client (note that this is much easier if you program the ESP8266 rather than the Arduino).  Your client needs to request the page from your server and then parse the returned data and extract the field you want.

While you are waiting for you network shield to arrive you could practice doing it on a Windows PC with Visual Studio, it will be easier to debug.