mysql - Previous/next button in PHP -

i´m pretty entirely new php, please bear me.

i´m trying build website running on cms called core. i'm trying make previous/next buttons cycle through tags rather entries. tags stored in database core_tags. each tag has own tag_id, number. i've tried changing excisting code thep previous/next buttons, keeps giving me 'warning: mysql_fetch_array() expects parameter 1 resource, null given in /home/core/functions/get_entry.php on line 50'.'

any appreciated.

get_entry.php:

<?php  $b = $_server['request_uri'];  if($entry) {     $b = substr($b,0,strrpos($b,"/")) . "/core/";     $id = $entry;     $isperma = true; } else {     $b = substr($b,0,mb_strrpos($b,"/core/")+6);     $id = $_request["id"]; }  $root = $_server['document_root'] . $b; $http = "http://" . $_server['http_host'] . substr($b,0,strlen($b)-5);  require_once($root . "user/configuration.php"); require_once($root . "themes/".$theme."/configuration.php"); require_once($root . "functions/session.php");  if(is_numeric($id)) {     $type = "entry"; } else {     $type = "page"; }  $id = secure($id);  if($type == "page") {     $data = mysql_query("select p.* core_pages p p.page_title = \"$id\"");     $page_clicks = 0;     while($p = mysql_fetch_array($data)) {         $url = $p["page_url"];         $path = $root . "user/pages/" . $url;         $page_clicks = $p['hits']+1;         require($path);     }      mysql_query("update core_pages p set     p.hits = $page_clicks     p.page_title = $id"); }  if($type == "entry") {  // queries dbase         $data_tags = mysql_query("select entry_id,entry_title core_entries entry_show = 1 order entry_position desc") or die(mysql_error());      $navarr=array();     while($tmparray = mysql_fetch_array($data_entries,mysql_assoc)){                 array_push($navarr,$tmparray['entry_id']);         }      function array_next_previous($array, $value) {     $index = array_search($value,$array);      //if user clicked view first entry     if($value == reset($array)){     $return['prev'] = end($array);     $return['next'] = $array[$index + 1];     //if user clicked view last entry     }else if($value == end($array)){     $return['prev'] = $array[$index - 1];     reset($array);     $return['next'] = current($array);     }else{         $return['next'] = $array[$index + 1];         $return['prev'] = $array[$index - 1];     }                 return $return;     }      $data = mysql_query("select e.* core_entries e e.entry_id = $id , e.entry_show = 1");     $entry_clicks = 0;     if(@mysql_num_rows($data) < 1) {         die("invalid id, no entry shown");     }     while($e = mysql_fetch_array($data)) {         $nextprevproject = array_next_previous($navarr,$id);          $entry_id       = $e['entry_id'];         $entry_title    = $e['entry_title'];         // date         $t              = $e["entry_date"];         $y              = substr($t,0,4);         $m              = substr($t,5,2);         $d              = substr($t,8,2);         $entry_date     = date($date_format,mktime(0,0,0,$m,$d,$y));         $entry_text     = $e['entry_text'];         $entry_extra1   = $e['entry_extra1'];         $entry_extra2   = $e['entry_extra2'];         $entry_client   = $e['entry_client'];         $entry_position = $e['entry_position'];         $entry_hits     = $e['hits']+1;         $entry_new      = $e['entry_new'];           if($entry_new == 1) {             $isnew = true;         } else {             $isnew = false;         }          if($nice_permalinks) {             $entry_perma = "$http".$entry_id;         } else {             $entry_perma = "$http"."?entry=$entry_id";         }          $data_e2t = @mysql_query("select e2t.tag_id core_entry2tag e2t e2t.entry_id = $entry_id");          $tag_str = "";              while($e2t = @mysql_fetch_array($data_e2t)) {                 $tag_id = $e2t["tag_id"];                 $data_tags = @mysql_query("select t.tag_text core_tags t t.tag_id = $tag_id");                     while($t = @mysql_fetch_array($data_tags)) {                         $tag_text = $t["tag_text"];                         $tag_str = $tag_str . "<a class=\"tag-link\" name=\"tag".$tag_id."\" href=\"#tag-"._encode($tag_text)."\">".$tag_text."</a>".$separator_tags;                     }             }              $entry_tags = substr($tag_str,0,strlen($tag_str)-strlen($separator_tags));          $layout_path = $root . "user/uploads/" . treat_string($entry_title) . "/layout.php";         if(is_file($layout_path) && (@filesize($layout_path) > 0)) {             require($layout_path);         } else {             require($theme_path . "parts/entry.php");         }     }      mysql_query("update core_entries e set     e.hits = $entry_hits     e.entry_id = $id");  }  if($isperma) { echo "<a class=\"index-link\" href=\"$http\">back index</a>"; } ?> 

you have not defined $data_entries, before using here:

    while($tmparray = mysql_fetch_array($data_entries,mysql_assoc)){                 array_push($navarr,$tmparray['entry_id']);         } 

that why descriptive error message. did mean use $data_tags?