PHP中將二維陣列中相同的欄位放在同一陣列中

<?php

$a = array( 

    0 =>array('student_id' => '1515925710', 'name' => 'test_1', 'phone' =>'13213791811', 'extension_number' => '15261', ), 

    1 =>array('student_id' => '1515925711', 'name' => 'test_2', 'phone' =>'13213791811', 'extension_number' => '66002', ), 

    14=>array('student_id' => '1515925712', 'name' => 'test_3', 'phone'=> '13213791811', 'extension_number' => '56431', ), 

    2 =>array('student_id' => '1515925713', 'name' => 'test_4', 'phone' => '13213791811','extension_number' => '54876', ), 

    3 =>array('student_id' => '1515925714', 'name' => 'test_5', 'phone' =>'13213791811', 'extension_number' => '07143', ), 

    13=>array('student_id' => '1515925715', 'name' => 'test_6', 'phone'=> '13213791811', 'extension_number' => '96949', ), 

    4 =>array('student_id' => '1515925716', 'name' => 'test_7', 'phone' =>'13213791811', 'extension_number' => '94755', ), 

    5 =>array('student_id' => '1515925717', 'name' => 'test_8', 'phone' =>'13213791811', 'extension_number' => '14171', ), 

    6 =>array('student_id' => '1515925718', 'name' => 'test_9', 'phone' =>'13213791811', 'extension_number' => '54876', ), 

    7 =>array('student_id' => '1515925719', 'name' => 'test_10', 'phone' =>'13213791811', 'extension_number' => '31354',), 

    8 =>array('student_id' => '1515925720', 'name' => 'test_11', 'phone' =>'13213791811', 'extension_number' => '66810', ), 

    9 =>array('student_id' => '1515925721', 'name' => 'test_12', 'phone' =>'13213791811', 'extension_number' => '34868', ), 

    10=>array('student_id' => '1515925722', 'name' => 'test_13', 'phone'=> '13213791811', 'extension_number' => '79523', ), 

    11=>array('student_id' => '1515925723', 'name' => 'test_14', 'phone'=> '13213791811', 'extension_number' => '59438', ), 

    12=>array('student_id' => '1515925724', 'name' => 'test_15', 'phone'=> '13213791811', 'extension_number' => '14780', ) 

); 

echo "未進行按欄位將陣列中的資料分開前";

print_r($a);

echo "按欄位分開後";

$arr=array();

foreach ($a as $nodeIndex ){

   $arr['student_id'][]=$nodeIndex['student_id'];

   $arr['name'][]=$nodeIndex['name'];

   $arr['phone'][]=$nodeIndex['phone'];

   $arr['extension_number'][]=$nodeIndex['extension_number'];

   $arr['extension_number'][]=$nodeIndex['extension_number'];

}

print_r($arr);

?>