PHP error. Why?

This error is likely happening because the code is trying to access an index of the array '$row' that does not exist. The code is checking for a value in the 'id' index of the array, but if there is no value, it will return NULL. This is causing the error, because the code is trying to access an array offset (the index it's trying to access) on a value of type NULL, which is not allowed. To fix this issue, you could use an isset() check to ensure that the index exists before trying to access it.