PHP ksort

1.例子一：

 <?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);