Php array combination

Php array combination $ set = array ('A', 'B', 'C', 'D ');

How to combine arrays? the output result is
A
B
C
B
AB
Ac
Ad
Bc
Bd

Reply to discussion (solution)

$ Set = array ('A', 'B', 'C', 'D'); $ res = $ set; for ($ I = 0, $ p = 1; $ I
 
  
Array
  
(
  
[0] =>
  
[1] => B
  
[2] => c
  
[3] => d
  
[4] => AB
  
[5] => bc
  
[6] => cd
  
)
  
 
  
 
  
  
  
   
  
$ Set = array ('A', 'B', 'C', 'D'); $ res = $ set; for ($ I = 0, $ p = 1; $ I
   
    
Array
    
(
    
[0] =>
    
[1] => B
    
[2] => c
    
[3] => d
    
[4] => AB
    
[5] => bc
    
[6] => cd
    
)
    
  
    
The outputs of the moderator are different from those of the landlord. I am not sure how the moderator combines the outputs of this array. my solution is as follows: no cd output is needed here.
    
     
  
    
  
    
  
    
array (size=10)  0 => string 'a' (length=1)  1 => string 'b' (length=1)  2 => string 'c' (length=1)  3 => string 'd' (length=1)  4 => string 'ab' (length=2)  5 => string 'ac' (length=2)  6 => string 'ad' (length=2)  7 => string 'bc' (length=2)  8 => string 'bd' (length=2)  9 => string 'cd' (length=2)
 
    
 
    
  
    
$set = array('a','b','c','d');$arr=array();for($j=0;$j
     
      $v){    for($i=$k;$i<=(3-$v);$i++){        if($v==$set[$i]) continue;        $arr[]=$v.$set[$i];    }}print_r($arr);
     
  
    
  
    
Array(    [0] => a    [1] => b    [2] => c    [3] => d    [4] => ab    [5] => ac    [6] => ad    [7] => bc    [8] => bd    [9] => cd)