<?php
echo '<pre>';
$sourceA = array(
	2	=> 'object 2',
	3	=> 'object 3',
	5	=> 'object 5'
);
$b = array(
	3	=> 'new instanceof object 3',
	5	=> 'new instanceof object 5',
	8	=> 'object 8',
	10	=> 'object 10',
	15	=> 'object 15',
);

/**
 * Print source arrays
 */
$a = $sourceA; // set default array
echo "Sources:\n";
echo "array a:\n";
print_r($a);
echo "\narray b:\n";
print_r($b);
echo "\n\n";

/**
 * array_merge result
 */
echo "Results:\n";
$a = array_merge($a, $b);
ksort($a);
echo "1. array_merge:\n";
print_r($a);

$a = $sourceA; // set default array
/**
 * (a + b) result
 */
$a = $a + $b;
ksort($a);
echo "\n\n2. summ (a + b):\n";
print_r($a);

$a = $sourceA; // set default array
/**
 * (b + a) result
 */
$a = $b + $a;
ksort($a);
echo "\n\n3. summ (b + a):\n";
print_r($a);
Sources:
array a:
Array
(
    [2] => object 2
    [3] => object 3
    [5] => object 5
)

array b:
Array
(
    [3] => new instanceof object 3
    [5] => new instanceof object 5
    [8] => object 8
    [10] => object 10
    [15] => object 15
)


Results:
1. array_merge:
Array
(
    [0] => object 2
    [1] => object 3
    [2] => object 5
    [3] => new instanceof object 3
    [4] => new instanceof object 5
    [5] => object 8
    [6] => object 10
    [7] => object 15
)


2. summ (a + b):
Array
(
    [2] => object 2
    [3] => object 3
    [5] => object 5
    [8] => object 8
    [10] => object 10
    [15] => object 15
)


3. summ (b + a):
Array
(
    [2] => object 2
    [3] => new instanceof object 3
    [5] => new instanceof object 5
    [8] => object 8
    [10] => object 10
    [15] => object 15
)

$a; $b;
$c = $a;
$a = $c + $b;

$this->_relatedObjects[$alias]= array_merge($this->_relatedObjects[$alias], $collection);

array_merge($this->_relatedObjects[$alias], $collection);

{assign var=i value=1}
<select>
    <option>Выбрать</option>
    {while $i <= $room.available}
    {assign var="mod" value=$i%100}
        <option>{$i} {if $mod > 4 && $mod < 20}номеров{elseif $mod==1}номер{else}номера{/if}</option>
        {$i++}
    {/while}
</select>

В общем было бы замечательно если бы modx из коробки поддерживал бы еще парочку шаблонизаторов на выбор)