how to use array_flip function with multidimensional array in php

$a=array("a",array("b","c","d"),"e","f");
array_flip($a);

Warning: array_flip() [function.array-flip]: Can only flip STRING and INTEGER values! 

function multidimensional_array_flip($array, $KeyToFlip, $KeyToFlip2=false, $Keep_Original_Key=false) {
$flipped_array=array();
foreach ($array as $Key=>$Value)
    { 
        if ($Keep_Original_Key) $Value[$Keep_Original_Key]=$Key;
         if (!is_string($Value[$KeyToFlip])) return false;

          if (is_string($KeyToFlip2))
        {
              if (!is_string($Value[$KeyToFlip2])) return false;

              $flipped_array[$Value[$KeyToFlip]][$Value[$KeyToFlip2]]=$Value;
        }

		
          else $flipped_array[$Value[$KeyToFlip]][]=$Value;
    }
return $flipped_array;
} 

	$d=multidimensional_array_flip($array_pages,1);

$a=array("A"=>"a",array("B"=>"b","C"=>"c","D"=>"d"),"E"=>"e","F"=>"f");
$extraArray=array();
function multiflip($a){
 	if(is_array($a)){
		foreach($a as $key=>$value){
			if(is_array($value)){
				$uniquekey="Array".rand(1,100);
				$extraArray[$uniquekey]=array_flip($value);	
				$a[$key]=$uniquekey;
			}
		}
		array_flip($a);
 

		foreach($a as $key=>$value){
			if(isset($extraArray[$value])){
				$a[$key]=$extraArray[$uniquekey];
			}
		}
	}

 
	return $a;
}
$a=multiflip($a);