如何将数组与json数据合并？ php，laravel

I have this json data:

"{"media_ids":[304,305,306]}"

and my array :

 { ["media_ids"]=> array(2) { [0]=> int(388) [1]=> int(389) } }

I attempted to merge them by doing this

$allData = array_merge($extra,json_decode($a,true));

But the result was my array data disappeared:

{ ["media_ids"]=> array(3) { [0]=> int(304) [1]=> int(305) [2]=> int(306) } }

You need to use array_merge_recursive

$allData = array_merge_recursive($extra,json_decode($a,true));