<?php

//There is a sorted array with unique elements of length n.
//Return all the PAIRS possible that when added gives you Y. 
//Sample Array: [10, 20, 30, 40, 50, 60, 70, 80, 90, 100] .  (Y=110)
//      0 ,  1,  2,  3,  4,  5,  6, 7,  8,  9        
//$arr = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100];
$arr = [-10, -5,0,10, 15, 30, 35, 45, 60, 70, 80, 90, 100];

$sumofPair = 20;
$start = 0;
$end = count($arr) -1;
$finalarr = [];
foreach($arr as $currentval){

    if($pair = binarySearch($currentval,$start,$end,$sumofPair,$arr)){
        $finalarr[] = $pair;
    }
}
print_r($finalarr);
function binarySearch($currentval,$start,$end,$sumofPair,$arr){

    while($start <= $end){

        $mid = $end - (int)($end - $start)/2;//9-0/2=4
        $tofind = $sumofPair - $currentval;

        if($tofind == $currentval) return;
            
        if($arr[$mid] < $tofind){
            $start = $mid + 1;
        }elseif($arr[$mid] > $tofind){
            $end = $mid - 1;
        }else{
            return [$currentval,$tofind];
        }
    }
    
}