Post variables and mysql queries

This may be a dumb question but here goes. I have been trying to use $_POST globals in sql queries. If I use the following query string it does not work $query="Select * from users where userid='$_POST['userid']'"; However, this works $userid=$_POST["userid"] $query="Select * from users where userid='$userid'"; Is there a mistake in my syntax? thanks, Luis R. Lebron Sigmatech, Inc