I'm trying to figure out how to write a php statement ...

here's a couple of url examples:

/displayimage.php?album=1&pos=0
/displayimage.php?album=1&pos=-1
/displayimage.php?album=1&pos=-2

What I want is if the "pos=0" is in the URL then I want to display:
this
If it's not, I want to display
this other thing

any suggestions?