Pass new tags followed by a table name and database handle and this function updates the table so that each tag becomes properly associated/its record created, eg:

table state:
tagname | relatedtags
-----------------------
blog | rss;personal
rss | blog
personal | blog

call (table is named tags):
updateTags("blog personal friends","tags",$db);

table state:
tagname | relatedtags
-----------------------
blog | rss;personal;friends
rss | blog
personal | blog;friends
friends | blog;personal

function updateTags($tags,$table,$db) {//category handling stuff
   include("recordExists.php");//see http://bigbold.com/snippets/posts/show/464
   $cats = explode(";",$tags);//parse tags into array cats
   foreach($cats as $cat) {//loop through cats
      $tmp = array($cat);//dummy array with cat in it
      $tmp = array_diff($cats,$tmp);//get everything in cats that is not cat
      if(!recordExists("tagname",$cat,$table,$db)) {//if there is no category by this name yet
         $tmp = implode(";",$tmp);//make tmp into semicolon-separated string
         $result = mysql_query("INSERT INTO ".$table." (tagname,relatedtags) VALUES ('$cat','$tmp')", $db) or die(mysql_error());//insert category into database
      } else {
         $result = mysql_query("SELECT relatedtags FROM ".$table." WHERE tagname='$cat'", $db) or die(mysql_error());//select already related tags
         $result = mysql_fetch_array($result);//get the result row as an array
         $result = explode(";",$result['relatedtags']);//parse relatedtags into result
         $result = array_merge($tmp,$result);//merge the cats in this $tags (without cat, hence tmp) with the ones that were in there already
         $result = array_unique($result);//strip duplicates
         $result = implode(";",$result);//make result into semicolon-separated string
         $result = mysql_query("UPDATE ".$table." SET relatedtags='$result' WHERE tagname='$cat'", $db) or die(mysql_error());//update category
      }//end if-else !recordExists
   }//end foreach cats
}//end function updateTags

In mySQL (from PHP), checks for the existance of a record based on id, passing the name of the id field followed by the id to check for, the table name, and the database resource, in that order.

function recordExists($id,$idval,$table,$db) {//check for id=idval in table and return TRUE or FALSE
   $result = mysql_query("SELECT * FROM ".$table." WHERE ".$id."='".$idval."'", $db) or die(mysql_error());
   if($row = mysql_fetch_array($result)) {//if we did return a record
      return 1;
   }//end if row
   return 0;
}//end fuction recordExists