How to check the query string of my current url for a specific key-value pair?

How I can identify in PHP if my current URL contains this text special_offer=12. For example, if my domain URL is http://ift.tt/2tFY86Q then echo something.

Alternatively, the domain can be http://ift.tt/2uftHkT

This text will always be the same: special_offer=12.