To calculate 'equilibrium indexes' for arrays. Read comments for details.

equilibrium_index.php

<?php
/** Problem found on Codility
 * A zero-indexed array A consisting of N integers is given. An equilibrium index of this array is any integer P such that 0 ≤ P < N and the sum of elements of lower indices is equal to the sum of elements of higher indices, i.e. 
 *     A[0] + A[1] + ... + A[P−1] = A[P+1] + ... + A[N−2] + A[N−1]. Sum of zero elements is assumed to be equal to 0. This can happen if P = 0 or if P = N−1.
 * For example, consider the following array A consisting of N = 8 elements:
 *     A[0] = -1, A[1] =  3, A[2] = -4, A[3] =  5, A[4] =  1, A[5] = -6, A[6] =  2, A[7] =  1
 * P = 1 is an equilibrium index of this array, because:
 *     A[0] = −1 = A[2] + A[3] + A[4] + A[5] + A[6] + A[7]
 * P = 3 is an equilibrium index of this array, because:
 *     A[0] + A[1] + A[2] = −2 = A[4] + A[5] + A[6] + A[7]
 * P = 7 is also an equilibrium index, because:
 *     A[0] + A[1] + A[2] + A[3] + A[4] + A[5] + A[6] = 0 and there are no elements with indices greater than 7.
 * P = 8 is not an equilibrium index, because it does not fulfill the condition 0 ≤ P < N.
 * Write a function:
 * function solution($A); that, given a zero-indexed array A consisting of N integers, returns any of its equilibrium indices. The function should return −1 if no equilibrium index exists.
 * For example, given array A shown above, the function may return 1, 3 or 7, as explained above.
 * Assume that: N is an integer within the range [0..100,000];
 * each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].
 * Complexity: expected worst-case time complexity is O(N); expected worst-case space complexity is O(N), beyond input storage (not counting the storage required for input arguments).
 * Elements of input arrays can be modified.
 */

function solution($A) {
    // write your code in PHP5.5
    $sum = 0;
    $length = count($A);
    $eq = -1;

    for($i = 0; $i < $length; $i++) {
    	if($i == 0) {
	    $copy = $A;
	    $end = array_splice($copy, 1, $length - 1);
	    $sum = array_sum($end);
	    if($sum == 0) {
	        $eq = $i;
            }
        } else if($i == $length - 1) {
	    $copy = $A;
	    $begin = array_splice($copy, 0, $length - 1);
	    $sum = array_sum($begin);
            if($sum == 0) {
                $eq = $i;
            }
        } else {
	    $copy1 = $copy2 = $A;
	    $pre = array_splice($copy1, 0, $i);
	    $post = array_splice($copy2, $i + 1, $length - 1);
            if(array_sum($pre) == array_sum($post)) {
                $eq = $i;
            }
        }
    }

    $sum = 0;
    return $eq;
}

solution([-1, 3, -4, 5, 1, -6, 2, 1]);