How to execute web code based on location

In php-mysql based site, I want to execute a small code depending on location. As a practical example think about the situation:

A site can have multiple advertisement providers. Those providers pay different for different country.

This site has two ad providers ADV1 pays high for European traffic but very less for Asian visitors. ADV2 shows ads only to Asian visitors but rate is higher than ADV1-Asian

So, the site should code something that will follow the algorithm:

If Visitor from Asia:

execute code ADV2 only to him

Else

Execute code Adv1 only to him

End If

Kindly help me to implement this.