PHP怎么合并二维数组

PHP如何合并二维数组
"arr": [
                {
                    "a": "XXX"
                },
                {
                    "a": "YYY"
                }
]

这是前段print_r出来的数据
我想改成：

"arr": [
                {
                     "XXX",
                    "YYY"
                }
]

------解决思路----------------------

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

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