function that takes an integer as an argument and returns "Even" for even numbers or "Odd" for odd numbers

isEvenOdd.php

if (! function_exists('isEvenOdd')) {
    /**
     * Determine if given number even or odd
     *
     * @param float|int $number
     *
     * @return string|InvalidArgumentException
     */
    function isEvenOdd($number)
    {
        if (! is_numeric($number)) {
            throw new InvalidArgumentException("Giving value must be a numeric {$number}", 100);
        }

        return ($number % 2 == 0)
            ? 'Even'
            : 'Odd';
    }
}

/**
 * @expectedException InvalidArgumentException
 */
public function test_is_even_odd()
{
    foreach ([-2, 0, 2, 4, 6] as $number) {
        $this->assertEquals(isEvenOdd($number), 'Even');
    }

    foreach ([-1, 1, 3, 5, 7] as $number) {
        $this->assertEquals(isEvenOdd($number), 'Odd');
    }

    // InvalidArgumentException
    isEvenOdd('foo');
}