似乎很简单:将所有pid值提取到自己的数组中，并通过array_unique运行:

Seems pretty simple: extract all pid values into their own array, run it through array_unique:

$uniquePids = array_unique(array_map(function ($i) { return $i['pid']; }, $holder));

从长远来看还是一样:

$pids = array();
foreach ($holder as $h) {
    $pids[] = $h['pid'];
}
$uniquePids = array_unique($pids);