<?php
function replace($arr) {
  if((count($arr)%2)==0) {
      $res=[];
      for($i=0; $i<count($arr); $i++) {
              if($i==(count($arr)/2-1)) $res[] = $arr[$i]+$arr[$i+1];
              else if($i!=(count($arr)/2)) $res[] = $arr[$i];
              return $res;
      }
    }else return $arr;
}

print_r(replace([7, 2, 2, 5, 10, 7]));
echo "<br>";
print_r(replace([-5,-5,10]));
echo "<br>";
print_r(replace([45, 23, 12, 33, 12, 453, -234, -45]));
echo "<br>";
print_r(replace([2,8]));
echo "<br>";
print_r(replace([-12, 34, 40, -5, -12, 4, 0, 0, -12]));
echo "<br>";
print_r(replace([9, 0, 15, 9]));
echo "<br>";
print_r(replace([-6, 6, -6]));
echo "<br>";
print_r(replace([26, 26, -17]));
echo "<br>";
print_r(replace([-7, 5, 5, 10]));
echo "<br>";