Additional Security Code authentication for user login

create an aditional authetication machanism along with the existing username and password to our PHP and MSSQL based web Application. example: the user enters his user name , password and the onetime secutiy code. the one secutiry code must be entered by looking to the random generated Alphabets matching the users static 4 digit pin stored in sql DB. for example is the users pin is 1234 then he will enter "QWER" username: .................... Password:..................... onetime security code: ..............(4Chars) QWERTYUIO----(Random change every time page loads) 1234567890 ----(static)