Building integrated forums to my website

Well, the forums are the first thing I'm doing but I'm having an error with my SQL.

SELECT * FROM categories_tbl LEFT JOIN forums_tbl ON (forums_tbl.parent_id = categories_tbl.category_id)
WHERE category_id=parent_id

The results of the var_dump: 

array(2) {
  [0]=>
  array(18) {
    ["category_id"]=>
    string(1) "1"
    [0]=>
    string(1) "1"
    ["category_name"]=>
    string(7) "General"
    [1]=>
    string(7) "General"
    ["category_parent_id"]=>
    string(1) "0"
    [2]=>
    string(1) "0"
    ["forum_id"]=>
    string(1) "1"
    [3]=>
    string(1) "1"
    ["forum_name"]=>
    string(8) "Welcome!"
    [4]=>
    string(8) "Welcome!"
    ["forum_created"]=>
    string(19) "2013-05-31 18:25:46"
    [5]=>
    string(19) "2013-05-31 18:25:46"
    ["forum_legend"]=>
    string(27) "Introduce yourself in here!"
    [6]=>
    string(27) "Introduce yourself in here!"
    ["parent_id"]=>
    string(1) "1"
    [7]=>
    string(1) "1"
    ["forum_createdby"]=>
    string(7) "Akhaten"
    [8]=>
    string(7) "Akhaten"
  }
  [1]=>
  array(18) {
    ["category_id"]=>
    string(1) "2"
    [0]=>
    string(1) "2"
    ["category_name"]=>
    string(9) "Computers"
    [1]=>
    string(9) "Computers"
    ["category_parent_id"]=>
    string(1) "0"
    [2]=>
    string(1) "0"
    ["forum_id"]=>
    string(1) "2"
    [3]=>
    string(1) "2"
    ["forum_name"]=>
    string(15) "PHP is Awesome!"
    [4]=>
    string(15) "PHP is Awesome!"
    ["forum_created"]=>
    string(19) "2013-05-31 18:25:46"
    [5]=>
    string(19) "2013-05-31 18:25:46"
    ["forum_legend"]=>
    string(18) "Let's all use PHP."
    [6]=>
    string(18) "Let's all use PHP."
    ["parent_id"]=>
    string(1) "2"
    [7]=>
    string(1) "2"
    ["forum_createdby"]=>
    string(5) "xLink"
    [8]=>
    string(5) "xLink"
  }
}