[Dayrion]

GetObjectPos is not defined like you did.
You have to create 3 variables x,y,z and uses those variables to store the position of the object.
GetObjectPos(objectID, x(store), y(store), z(store)

PHP Code:

new Float:x, Float:y, Float:z;
GetObjectPos(objectid, x, y,z)
If( x == ... &&/|| y=...)
{
    //Blabla
} 


NB: I'm on my phone, I can't do better :$
Edit: he was more faster than me 😐