function recursive($array, $level = 0)
{
    foreach($array as $key => $value)
	{
        
		if (!is_array($value) && $key == "tag") { echo str_repeat(" ", $level), "[".$value."]", ''; }
		
		//If $value is an array.
        if(is_array($value))
		{
			//We need to loop through it.
            recursive($value, $level + 1);
        } 
		else
		{
            //It is not an array, so print it out.			
			if ($key == "tag") { echo "[/".$value."]", '<br>'; }
						
        }
    }
}

$tree_2 = recursive($tree_1);

echo $tree_2;