array_merge() is useful to merge arrays but it is confusing which parameter takes precedence. Also it is different from the method with the plus operator. ( e.g. $arr1 + $arr2 ). It could be more like join two array elements. So it may be used to append or pre-pend numerically indexed arrays.

sample_array_merge.php

<?php
$array1 = array( 'a', 'b', 'c' ); 
$array2 = array( 'd', 'e', );

$result1 = array_merge( $array2, $array1 );
$result2 = array_merge( $array1, $array2 );

echo '<pre>' . print_r( $result1, true ) . '</pre>';
echo '<pre>' . print_r( $result2, true ) . '</pre>';