Is there a way to check to see if a variable/index exists



I am trying to write a code that first checks to see if a value is available for a variable. If it is not available I want to run a part of my script that enters the information. If it is available then I want to move on to the next step. How do I do that? My current code keeps kicking out a notice for an undefined index, which makes sense because the script has yet to see the variable in the script. It doesn't get posted until after the script executes.
nevermind! I discovered the isset() function! It works! I used the isset() function to check for the variable first. Now it works great!