<?php
$a = array('r'=>1,2,3,4);
var_dump('a',$a);
$b = array('r'=>5,6,7,8);
var_dump('b',$b);
$c = array('r'=>5,6,7,8,9);
var_dump('c',$c);

var_dump('a+b',$a+$b);
var_dump('a+c', $a+$c);

var_dump('amb', array_merge($a, $b));
var_dump('amc', array_merge($a, $c));

string 'a' (length=1)
array
  'r' => int 1
  0 => int 2
  1 => int 3
  2 => int 4
string 'b' (length=1)
array
  'r' => int 5
  0 => int 6
  1 => int 7
  2 => int 8
string 'c' (length=1)
array
  'r' => int 5
  0 => int 6
  1 => int 7
  2 => int 8
  3 => int 9

string 'a+b' (length=3)
array
  'r' => int 1
  0 => int 2
  1 => int 3
  2 => int 4

string 'a+c' (length=3)
array
  'r' => int 1
  0 => int 2
  1 => int 3
  2 => int 4
  3 => int 9

string 'amb' (length=3)
array
  'r' => int 5
  0 => int 2
  1 => int 3
  2 => int 4
  3 => int 6
  4 => int 7
  5 => int 8

string 'amc' (length=3)
array
  'r' => int 5
  0 => int 2
  1 => int 3
  2 => int 4
  3 => int 6
  4 => int 7
  5 => int 8
  6 => int 9