PHP-合并两个一维数组

<?php
 $array1  = array( 0  =>  'zero_a' ,  2  =>  'two_a' ,  3  =>  'three_a' );
 $array2  = array( 1  =>  'one_b' ,  3  =>  'three_b' ,  4  =>  'four_b' );
 $result  =  $array1  +  $array2 ;
 var_dump ( $result );
 ?>