php - Inserting data into 3 tables in 1 query -

basically have site tag-it system similar 1 on site, table structures follows

topics      topic_tags    tags topic_id    tag_id        tag_id topic_data  topic_id      tags 

now trying run series of queries when topic posted, enters relevant topic data topic table(currently working), puts tags tags table(currently working), puts tag_id tags table topic_tags table in rows linked topic_ids(not working currently)

right it's not entering tag_ids topic_tags tags table, it's entering last tags info row , not storing associated topic_ids

here code

$tags = isset($_post['tags']) ? $_post['tags'] : null;  if (is_array($tags)) { foreach ($tags $t) {     // checking duplicate      $sql_d = "select * tags tags='$t'";        $res=mysql_query($sql_d);       $res = mysql_num_rows($res);     if($res<1)     {     // escape $t before inserting in db     $sql = "insert tags (tags) values('$t')";     mysql_query($sql);     }  } } else { echo 'invalid tag'; }  $tag_id = isset($_post['tag_id']) ? $_post['tag_id'] : null;  if (is_array($tag_id)) { foreach ($tag_id $tid) {      // escape $t before inserting in db     $sql = "insert topic_tags (tag_id) values('$tid')";     mysql_query($sql);     }  }  $sql="insert topic_tags (tag_id)values(last_insert_id())"; $result=mysql_query($sql);   $topic_data= htmlentities($_post['topic_data']); $posted_by = $_session['user_id']; $posted = "date_add(now(),interval 2 hour)"; $invisipost = isset($_post['invisipost']) ? $_post['invisipost'] : 0 ;  if (($topic_data==""))  echo "<h2>opps...</h2><p>you did not fill out required fields.</p>";  else  $sql="insert topics(topic_data, posted_by, posted, invisipost)values('$topic_data', '$posted_by', $posted, $invisipost)"; $result=mysql_query($sql);  if($result){  $sql="insert topic_tags (topic_id)values(last_insert_id()) topic_tags.tag_id='". $_get['tags'] ."'"; $result=mysql_query($sql); 

edit

would better add topics data first, tags, topic_tags info??

i think should insert topic_tags in 1 step in 'foreach' loop. last query

$sql="insert topic_tags (topic_id)values(last_insert_id()) topic_tags.tag_id='". $_get['tags'] ."'" 

is neither insert nor update , wrong

(edit) need in lines of below:

foreach ($tag_id $tid) {      // escape $t before inserting in db     $sql = "insert topic_tags (topic_id,tag_id) values($topic_id,$tid)";     mysql_query($sql); }