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Write a function named factorial that accepts an integer $n as a parameter and returns the factorial of $n, or $n!. A factorial of an integer is defined as the product of all integers from 1 through that integer inclusive.

For example, the call of factorial(4) should return 1 * 2 * 3 * 4, or 24. The factorial of 0 and 1 are defined to be 1. If passed a parameter value that is negative, your function should throw an exception with the message, "Error: parameter value must be non-negative."

You may assume that the result factorial can fit in the range of type Integer.

Function: Write a PHP function as described, not a complete program.

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