Here's a more complete solution for Google's Example Coding/Engineering Interview video at https://youtu.be/XKu_SEDAykw

google_eng_example.php

<?php
// PHP ^7.1
$ar1 = [1,2,5,9,-1,3,4,5];	// test array #1
$ar2 = [1,2,4,4,9,3,4,5,3];	// test array #2

/**
 * Function that finds indices of x-complements to an element in an array
 *
 *   @param	array		arr	an array of integer/float numbers
 *   @param	int		sum	the base for complements
 *
 *   @return	array|null		an array that shows indices of element and it's complements if found any, null otherwise
 */
function find_complements(array $arr, int $sum): ?array {
	// hash table of x-complements found
	$hash_comp = [];
	// hash table of elements
	$hash_elem = [];
	// final output
	$res = [];

	// iterating over each element: Θ(n)
	foreach($arr as $idx => $elem){
		$comp = $sum - $elem;
		// if complement is found in complements' hash table, then we have already met an element that its complemet matches current element. Hence, this is a solution.
		if (isset($hash_comp[$comp]))
		{
			isset($res[$elem]) ?? $res[$elem] = [];
			// filling hash tables 
			$hash_comp[$elem][] = $idx;
			$hash_elem[$elem][] = $idx;
			// addint current element and its complements to the final results "BY REFERENCE", to let hash tables be completed on later iteration. 
			$res[$elem]['element_idx'] = &$hash_elem[$elem];
			$res[$elem]['complement_idx'] = &$hash_comp[$comp];
			// keep only the smallest results since each result carries the complements too.
			switch ($elem <=> $comp){
				case -1:{
					unset($res[$comp]);
					break;
				}
				case 1:{
					unset($res[$elem]);
					break;
				}
			}
			continue;
		}
		// filling hash tables
		$hash_comp[$elem][] = $idx;
		$hash_elem[$elem][] = $idx;
	}
	
	return $res ?? null;
}

// commencing test
var_dump(find_complements($ar1,8));
var_dump(find_complements($ar2,8));