merge

<?php

   function addchild(&$tree,$prefix,$lvl,$idx){

      if($idx<strlen($lvl)){

           if(!isset($tree[$prefix.'_'.substr($lvl,0,$idx+1)])){
             $tree[$prefix.'_'.substr($lvl,0,$idx+1)] = array();
           }
           addchild($tree[$prefix.'_'.substr($lvl,0,$idx+1)],$prefix,$lvl,$idx+1);
      }
   }


$str1 = "Node_1.Node_11.Node_111.Node_1111.Node_11111";
$str2 = "Node_1.Node_12.Node_1121.Node_11211.Node_12111";
$str3 = "Node_1.Node_12.Node_123.Node_1231.Node_12311";
$str4 = "Node_2.Node_21.Node_211.Node_2111.Node_21111";
$all = $str1.'|'.$str2.'|'.$str3.'|'.$str4;

$tree=array();

$lvl1 = explode('|',$all);

foreach($lvl1 as $str)
{
    $lvl2 = explode('.',$str);
    foreach($lvl2 as $node)
    {
      $temp = explode('_',$node);
      addchild($tree,$temp[0],$temp[1],0);

    }

}
echo '<pre>';
print_r($tree);
echo '</pre>';

?>