$arr = [....]; //你上面的数组
$newArr = [];
foreach ($arr as $item) {
    $newArr[$item['p_id']][] = $item;
}
print_r($newArr);

用array_column就可以了

array_column 才是最佳选择
<?php
// 表示由数据库返回的可能记录集的数组
$a = array(
array(

'id' => 5698,
'first_name' => 'Bill',
'last_name' => 'Gates',

),
array(

'id' => 4767,
'first_name' => 'Steve',
'last_name' => 'Jobs',

)
array(

'id' => 3809,
'first_name' => 'Mark',
'last_name' => 'Zuckerberg',

)
);

$last_names = array_column($a, 'last_name');
print_r($last_names);
?>
输出：
Array
(
[0] => Gates
[1] => Jobs
[2] => Zuckerberg
)

$arr = [
    0 => [
        'id'          => 11,
        'name'        => '奥巴马',
        'winning'     => 5,
        'title_honor' => 5,
        'photo'       => '',
        'classid'     => 10186,
        'site_id'     => 313,
        'listorder'   => 0,
        'p_id'        => 5,
    ],

    '1' => [
        'id'          => 10,
        'name'        => '习近',
        'winning'     => '三等奖',
        'title_honor' => '荣誉科学',
        'photo'       => 'themes/313/userfiles/images/2016/12/6/source/8ops1ho3dm9i04u.jpg',
        'classid'     => 10186,
        'site_id'     => 313,
        'listorder'   => 0,
        'p_id'        => 6,
    ],

    '2' => [
        'id'          => 9,
        'name'        => '奥巴马',
        'winning'     => '二等',
        'title_honor' => '2013年度同济大学医学院“特色导师”称号',
        'photo'       => 'themes/313/userfiles/images/2016/12/6/source/u0g8xhd1gzphkpf.jpg',
        'classid'     => 10186,
        'site_id'     => 313,
        'listorder'   => 0,
        'p_id'        => 5,
    ],

    '3' => [
        'id'          => 8,
        'name'        => '奥巴马',
        'winning'     => '一等奖',
        'title_honor' => '先锋队',
        'photo'       => 'themes/313/userfiles/images/2016/12/6/source/qtqukvv4birtkdc.jpg',
        'classid'     => 10186,
        'site_id'     => 313,
        'listorder'   => 0,
        'p_id'        => 5,
    ],
];

$column = array_column($arr, 'p_id');
echo '<pre>';
var_export($column);

• 输出结果如下

• array (

• 0 => 5,

• 1 => 6,

• 2 => 5,

• 3 => 5,

• )