PHP如何合并二维数组

$s =<<< TXT{"arr": [                {                    "a": "XXX"                },                {                    "a": "YYY"                }]}TXT;$t = json_decode($s, 1);array_walk($t['arr'], function(&$v) {$v = current($v);});print_r($t);echo json_encode($t);

Array(    [arr] => Array        (            [0] => XXX            [1] => YYY        )){"arr":["XXX","YYY"]}

$t['arr'] = array_map('current', $t['arr']);echo json_encode($t);

<?php$o = <<<FDIPZONE{"arr": [                {                    "a": "XXX"                },                {                    "a": "YYY"                }]}FDIPZONE;$arr = json_decode($o, true);$tmp = array();foreach($arr['arr'] as $k=>$v){    array_push($tmp, $v['a']);}$result = array('arr'=>array($tmp));header('content-type:application/json');echo json_encode($result, JSON_UNESCAPED_UNICODE|JSON_PRETTY_PRINT);?>