The function in_array() requires the second argument ($haystack) to be an array, but it is currently a null value.

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This means that you need to provide an array as the second argument to the inarray() function, which can be searched for a specific value. You cannot pass a null value as the haystack parameter. If you want to check if a value exists in an array, you need to pass that array as the second argument to the inarray() function.