<?php

 /**

  * 根据 c1 对元素排序

  */

 $arrays = [

     'b' => [

         'c1' => 10,

         'c2' => 5,

     ],

     'a' => [

         'c1' => 5,

         'c2' => 8,

     ],

     'c' => [

         'c1' => 9,

         'c2' => 10,

     ],

     'e' => [

         'c1' => 5,

         'c2' => 10,

     ],

 ];

 $t = [];

 $c = 'c1';

 foreach($arrays as $key => $array){

     $cv = $array[$c];

     if(!isset($t[$cv])){

         $t[$cv] = [];

     }

     $t[$cv][] = $key;

 }

 ksort($t);

 print_r($t);

 foreach($t as $v){

     if(is_array($v)){

         foreach($v as $y){

             $s[$y] = $arrays[$y];

         }

     }else{

         $s[$y] = $arrays[$y];

     }

 }

 print_r($s);