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There are several ways to resolve an Undefined array key issue:

1. Check if the key is defined: Make sure that the key you are trying to access is actually defined in the array. You can check this by using the arraykeyexists() function.

2. Use isset(): You can use the isset() function to check if an array key is set before trying to access it.

3. Use default values: You can provide a default value for the key if it is not set, using the null coalescing operator (??) or the ternary operator.

4. Check your code logic: Check your code logic and ensure that you are using the correct array keys and indices.

5. Debugging: Use debugging tools such as vardump(), printr() or xdebug to explore the array structure and see which key is causing the issue.

6. Turn on error reporting: Turn on error reporting to help identify where the issue is coming from.