<?php



function reduceFilenameArray($array_1 = array()) {
   $array_2 = array();
   foreach( $array_1 as $val) {
      // If appears as a directory name, add a filename to the end
      if (strpos(basename($val), '.') !== false){
         $newVal = dirname($val);
      } else {
         $newVal = dirname($val.'/.');
      }

      // filenames without directories should be kept according to the example
      $newVal = $newVal == '.' ? $val : $newVal;

      // Build the new array by adding unique values only
      if (!in_array($newVal, $array_2)) {
         $array_2[] = $newVal;
      }
   }

   return $array_2;
}



$array1 =Array
(
	'folder1/folder2',
	'folder1/folder2/images1.jpg',
	'folder1/folder2/images2.jpg',
	'folder1/folder3',

	'folderA',
	'folderA/folderA1/folderA2/A.jpg',
	'folderA/folderA3/someFile.jpg',
	
	'folderB/folderB/folderB',
	'folderB/folderB/folderB/somefile.txt',
	'folderB/folderB/folderB/somefolder',
	
	'folderC/file1.pdf',
	'folderC/file2.pdf',
	
	'folderD/.htaccess',
);

print_r(reduceFilenameArray($array1));


/* Suggested Solution should be.
Array
(
    [0] => folder1/folder2
    [1] => folder1/folder3
    [2] => folderA
    [3] => folderB/folderB/folderB

    [4] => folderC/file1.pdf
    [5] => folderC/file2.pdf

    [6] => folderD/.htaccess
)
*/

?>

Array
(
    [0] => folder1/folder2
    [1] => folder1/folder3
    [2] => folderA
    [3] => folderA/folderA1/folderA2
    [4] => folderA/folderA3
    [5] => folderB/folderB/folderB
    [6] => folderB/folderB/folderB/somefolder
    [7] => folderC
    [8] => folderD
)