Получение списка всех подкаталогов в текущем каталоге

Попробуйте это

 public function getNextVal(&$array, $curr_val){

    foreach($array as $k=>$v){
        if($v['your_key'] == $curr_val){
            if(isset($array[$k+1]))
                return $array[$k+1];
            else
                return $array[0];
        }
    }

}

public function getPrevVal(&$array, $curr_val){

    foreach($array as $k=>$v){
        if($v['your_key'] == $curr_val){
            if(isset($array[$k-1]))
                return $array[$k-1];
            else
                return end($array);
        }
    }
}

для массива следующим образом:

array (size=3)
0 => 
array (size=11)
  'id' => string '21' (length=2)
  'cat' => string '1' (length=1)
  'gal' => string '1' (length=1)
  'type' => string 'image' (length=5)
  'name' => string 'chalk_art_dies-irea_2nd_pic' (length=27)
  'permalink' => string 'chalk-art-dies-irea-2nd-pic' (length=27)
  'url' => string 'rxNsPoEiJboJQ32.jpg' (length=19)
  'content' => string '' (length=0)
  'date' => string '1432076359' (length=10)
  'order' => string '20' (length=2)
  'views' => string '0' (length=1)
 1 => 
   array (size=11)
  'id' => string '10' (length=2)
  'cat' => string '1' (length=1)
  'gal' => string '1' (length=1)
  'type' => string 'image' (length=5)
  'name' => string '3dchalkart' (length=10)
  'permalink' => string '3dchalkart' (length=10)
  'url' => string 's57i5DKueUEI4lu.jpg' (length=19)
  'content' => string '' (length=0)
  'date' => string '1432076358' (length=10)
  'order' => string '9' (length=1)
  'views' => string '0' (length=1)
  2 =>