array中按照key排序的问题？

<code>$test=array(60) {
 [0]=> object(stdClass)#14 (3) { ["id"]=> string(3) "609" ["title"]=> string(19) "Physical Mechanisms" ["publish_time"]=> string(10) "6 MAY 2016" }
 [1]=> object(stdClass)#14 (3) { ["id"]=> string(3) "610" ["title"]=> string(20) "Engineering Substrate" ["publish_time"]=> string(10) "2 MAY 2016" }
  ...}
 

</code>

<code>$test=array(60) {
 [0]=> object(stdClass)#14 (3) { ["id"]=> string(3) "609" ["title"]=> string(19) "Physical Mechanisms" ["publish_time"]=> string(10) "6 MAY 2016" }
 [1]=> object(stdClass)#14 (3) { ["id"]=> string(3) "610" ["title"]=> string(20) "Engineering Substrate" ["publish_time"]=> string(10) "2 MAY 2016" }
  ...}
 

</code>

<code><?php
function _usort($a, $b) {
    if ( strtotime($a->publish_time) < strtotime($a->publish_time) ) {
        return -1;
    } else if( strtotime($a->publish_time) > strtotime($a->publish_time) ){
        return 1;
    } else {
        return 0;
    }
}

usort($arr, '_usort');</code>