detecting POST variables

Hello, I have been dealing with different form proccessing PHP scripts. When I start a new job I usually have a script that just takes the form POST values and prints them to the screen, so I know what I'm working with. Now I have a question say the form has 1 field named "FirstName". If the field is left blank, sometimes my script will print the key and NO value, or it will just print out nothing, ie NO key or value. any ideas. Could it be the server (IIS or Unix/Linux) of the form and the server (IIS or Unix/Linux) of the script.? Since I do jobs for different people and environments any combination of servers are possible. The reason, for this post is that I don't know if I should always put code to check for validilty using isset all the time or should I put it in just in case the script gets moved. I want to pinpoint this so that I can write more effective code.