This error means that within your code, there is a variable or constant that has no value assigned to it. But you may be trying to use the values obtained through the user form in your PHP code.
The error can be avoided by using the isset() function. This function will check whether the index variables are assigned a value or not, before using them.
Based on the error and code lines you share, it seems the
is not defined. i.e.: Index '
' does not exists in the variable
You have the entire code. Use DEBUGGER and step through to see the same and then find why so, handle it. If
helps, go ahead and use it based on the flow.
PHP isset() Function
PHP: Debugging in PHP - Manual