$属性？这就是你的问题所在。

可以找到一个例子 here （引用为完整性）

 <？php 
 / ** 
 *返回一维数组的幂组，即二维数组。 
 * [a，b，c]  - > [a]，[b]，[c]，[a，b]，[a，c]，[b，c]，[a，b，c]] 
 * / 
函数powerSet（$ in，$ minLength = 1）{
 $ count = count（$ in）; 
 $ members = pow（2，$ count）; 
 $ return = array（）; 
（$ i = 0; $ i< $ members; $ i ++）{
 $ b = sprintf（％0。$ count。b，$ i）; 
 $ out = array（）; 
（$ j = 0; $ j  if（$ b {$ j} =='1'）$ out [] = $ in [$ j ]。 
} 
 if（count（$ out）> = $ minLength）{
 $ return [] = $ out; 
} 
} 
 return $ return; 
} 
 

I have a Relational Schema with attributes (A B C D). I have a set of Functional Dependencies with me too.

Now I need to determine the closure for all the possible subsets of R's attributes. That's where I am stuck. I need to learn how to find subsets (non-repeating) in PHP.

My Array is stored like this.

$ATTRIBUTES = ('A', 'B', 'C', 'D').

so my subsets should be

$SUBSET = ('A', 'B', 'C', 'D', 'AB', 'AC', AD', 'BC', 'BD', 'CD', 'ABC', 'ABD', 'BCD', 'ABCD')

The code shouldn't be something big but for some reason I can't get my head around it.

You wish for the power set of $attributes? That is what your question implies.

An example can be found here (quoted for completeness)

<?php 
/** 
* Returns the power set of a one dimensional array, a 2-D array. 
* [a,b,c] -> [ [a], [b], [c], [a, b], [a, c], [b, c], [a, b, c] ]
*/ 
function powerSet($in,$minLength = 1) { 
   $count = count($in); 
   $members = pow(2,$count); 
   $return = array(); 
   for ($i = 0; $i < $members; $i++) { 
      $b = sprintf("%0".$count."b",$i); 
      $out = array(); 
      for ($j = 0; $j < $count; $j++) { 
         if ($b{$j} == '1') $out[] = $in[$j]; 
      } 
      if (count($out) >= $minLength) { 
         $return[] = $out; 
      } 
   } 
   return $return; 
}