怎么样通过PHP实现查询两个数组中不同元素？

<?php
 $a = array(
 "max_allow_dialogs",
 "livechat_server_ip",
 "livechat_service_time",
 "abort_zh_cn",
 "abort_zh_tw",
 "abort_en_usa",
 "welcome_zh_cn",
 "welcome_zh_tw",
 "welcome_en_usa",
 "timeout_zh_cn",
 "timeout_zh_tw",
 "timeout_en_usa",
 "absence_zh_cn",
 "absence_zh_tw",
 "absence_en_usa"
 );
 $b = array(
 "max_allow_dialogs",
 "livechat_server_ip",
 "livechat_service_time",
 "abort_zh_cn",
 "abort_zh_tw",
 "abort_en_usa",
 "welcome_zh_cn",
 "welcome_zh_tw",
 "welcome_en_usa",
 "timeout_zh_cn",
 "timeout_zh_tw",
 "timeout_en_usa",
 );
 $c = array_merge(array_diff($a,$b),array_diff($b,$a));
 print_r($c);
?>

Array
(
 [0] => absence_zh_cn
 [1] => absence_zh_tw
 [2] => absence_en_usa
)