<?php
print '<pre>';
$a = array(
    2 => 'object'    
);

$b = array(
    2 => 'new object',
    5 => 'object',
    7 => 'object',
) + $a;

print_r($b);

Array
(
    [2] => new object
    [5] => object
    [7] => object
)