The error "Undefined index: (name of the file input)" in PHP usually occurs when the form that is used to upload the file is not properly configured.
In Laravel, an "Undefined offset" error typically occurs when trying to access an array index that does not exist.
Read this JavaScript tutorial and get relevant information about the differences between null and undefined data types. Find out their similarities as well.
The PHP error "Notice: Use of undefined constant" means that you are trying to use a constant that has not been defined yet.