Group consecutively repeated values of a flat array and concatenate the values within each group

How can the array be separated into groups of identical elements (concatenated chars). For example, I have this array:

Array(
    [0] => 1
    [1] => 1
    [2] => 1
    [3] => 2
    [4] => 2
    [5] => 1
)

and want to group all identical numbers in only one element, bringing the together with concatenation to get something like this:

Array(
    [0] => 111
    [1] => 22
    [2] => 1
)

To group all identical elements by concatenating them together (will not work for concatenating just the neighboring identical elements)

$arr = array (1,1,1,2,2,1);
$sum = array_count_values($arr);
array_walk($sum, function(&$count, $value) { 
  $count = str_repeat($value, $count); 
});
print_r($sum);

Output

Array ( [1] => 1111 [2] => 22 )

Or for concatenating just the neighboring identical elements

$prev=null; 
$key=0; 
foreach ( $arr as $value ) { 
  if ($prev == $value){ 
    $res[$key] .= $value;
  } else { 
    $key++; 
    $res[$key] = $value; 
    $prev=$value; 
  } 
}
print_r($res);

Output

Array ( [1] => 111 [2] => 22 [3] => 1 )