PHP: Equal Sides Of An Array

I’m trying to build a function that returns the index of an array where equal sides of the array sum to the same value.

I can get all tests to pass apart from this one:
$this->assertSame(6,find_even_index([10,-80,10,10,15,35,20]));

I don’t understand why this test has been added by Codewars when it doesn’t seem correct? Index 6 is the last item, and doesn’t have a right side to equate.

Challenge was taken from Codewars:
https://www.codewars.com/kata/5679aa472b8f57fb8c000047/solutions/php

Code:

$numbers = [10,-80,10,10,15,35,20];

echo getEqualSidesIndex($numbers) . PHP_EOL;

function getEqualSidesIndex(array $numbers) : int {
    foreach ($numbers as $key => $value) {
        if ($key < 1) {
            continue; 
        }
    
        $itemsBeforeCurrent = array_slice($numbers, 0, $key - 1);
        $itemsAfterCurrent = array_slice($numbers, $key, count($numbers) - 1);

        if (array_sum($itemsBeforeCurrent) == array_sum($itemsAfterCurrent)) {
            return $key - 1;
        }
    }

    return -1;
}

Tests:

class FindEvenIndexTest extends TestCase
{
    public function testIt() {
      $this->assertSame(3,find_even_index(array(1,2,3,4,3,2,1)));
      $this->assertSame(1,find_even_index([1,100,50,-51,1,1]));
      $this->assertSame(-1,find_even_index([1,2,3,4,5,6]));
      $this->assertSame(3,find_even_index([20,10,30,10,10,15,35]));
      $this->assertSame(0,find_even_index([20,10,-80,10,10,15,35]));
      $this->assertSame(6,find_even_index([10,-80,10,10,15,35,20]));
      $this->assertSame(-1,find_even_index(range(1,100)));
      $this->assertSame(0,find_even_index([0,0,0,0,0]),"Should pick the first index if more cases are valid");
      $this->assertSame(3,find_even_index([-1,-2,-3,-4,-3,-2,-1]));
      $this->assertSame(-1,find_even_index(range(-100,-1)));

    }
}

Instructions:

You are going to be given an array of integers. Your job is to take
that array and find an index N where the sum of the integers to the
left of N is equal to the sum of the integers to the right of N. If
there is no index that would make this happen, return -1.

For example:

Let’s say you are given the array {1,2,3,4,3,2,1}: Your function will
return the index 3, because at the 3rd position of the array, the sum
of left side of the index ({1,2,3}) and the sum of the right side of
the index ({3,2,1}) both equal 6.

Let’s look at another one. You are given the array {1,100,50,-51,1,1}:
Your function will return the index 1, because at the 1st position of
the array, the sum of left side of the index ({1}) and the sum of the
right side of the index ({50,-51,1,1}) both equal 1.

Last one: You are given the array {20,10,-80,10,10,15,35} At index 0
the left side is {} The right side is {10,-80,10,10,15,35} They both
are equal to 0 when added. (Empty arrays are equal to 0 in this
problem) Index 0 is the place where the left side and right side are
equal.

Note: Please remember that in most programming/scripting languages the
index of an array starts at 0.

Input: An integer array of length 0 < arr < 1000. The numbers in the
array can be any integer positive or negative.

Output: The lowest index N where the side to the left of N is equal to
the side to the right of N. If you do not find an index that fits
these rules, then you will return -1.

Note: If you are given an array with multiple answers, return the
lowest correct index.