I hav been tryin 2 create a cookie from a php script, bt the cookie is not being created!

I hav search loads and loads of forums and web-pages 2 try 2 find the answer and dispite learning a lot about creating cookies using php I hav not been able 2 find a solution 2 whatever is causing my problem.

So in wh@ feals like desperation I come to AO in order 4 sum1 2 tell me wh@ I hav been doing wrong . The piece of code that I am tryin 2 create to cookie using is shown below:

PHP Code:
setcookie("example.co.uk.login.usr",$_POST["usr"]); 
Any & all assistance will b greatfully recieved.

Thank You,
TTAYO