How can I show error message inside a form? - php

Question

The page only show message when I click submit button, but now I want the message show inside the form after click submit button.How can I add some code or change the code following to make the message can run inside the form?
Here is my php code:
<?php
function topic_go($id){
echo "<meta http-equiv=\"refresh\" content=\"0;url=main_forum.php?act=topic&id=".$id."\">";
}
$id = $_GET['id'];
if(!$_SESSION['sign_in']){
$sql4= "SELECT * FROM categories WHERE level <".$_SESSION['userlevel']."+1";
$res4= mysql_query($sql4) or die (mysql_error());
$row4 = mysql_fetch_assoc($res4);
$sql5= "SELECT * FROM sub_categories WHERE sub_id ='".$id."'";
$res5 = mysql_query($sql5) or die (mysql_error());
$row5 = mysql_fetch_assoc($res5);
echo "<script type=\"text/javascript\">";
echo "alert('Please Login To Create Topic!');";
echo "window.location='main_forum.php?act=forum&id=".$row5['sub_id']."'";
echo "</script>";
}else{
if($id){
$sql="SELECT * FROM sub_categories WHERE sub_id = '".$id."'";
$res = mysql_query($sql) or die(mysql_error());
if(mysql_num_rows($res) == 0){
echo "The forum you are trying to create a topic on, does not exist!\n";
}else{
$row1 = mysql_fetch_assoc($res);
if ($row1['level'] == 1 && $admin_user_level == 0){
echo "You are not an administrator, you cannot post on this forum";
}else {
if (!$_POST['submit']) {
echo "<table bgcolor=\"#CFFAE4\" cellspacing=\"10\" align=\"center\">\n";
echo "<form method=\"post\" action=\"./main_forum.php?act=create&id=".$id."\">\n";
echo "<tr><td>Forum Sub Category</td><td><select name=\"cat\" style=\"font-size:16px;\">\n";
$sql2= "SELECT * FROM categories WHERE level <".$admin_user_level."+1";
$res2= mysql_query($sql2) or die (mysql_error());
while($row = mysql_fetch_assoc($res2)){
$sql3= "SELECT * FROM sub_categories WHERE sub_cid = '".$row['cat_id']."'";
$res3 = mysql_query($sql3) or die (mysql_error());
echo "<option value=\"0\">".$row['cat_name']."</option>\n";
while($row2 = mysql_fetch_assoc($res3)){
$selected = ($row2['sub_id'] == $id) ? " SELECTED" : "";
echo "<option value=\"".$row2['sub_id']."\"".$selected."> ".$row2['sub_name']."</option>\n";
}
}
echo "</select></td></tr>\n";
echo "<tr><td valign=\"top\">Topic Title</td><td><textarea name=\"title\" style=\"width:400px;height:50px;font-size:16px\"></textarea></td></tr>\n";
echo "<tr><td valign=\"top\">Message</td><td><textarea name=\"message\" style=\"width:500px;height:300px;font-size:20px;\"></textarea></td></tr>\n";
echo "<tr><td colspan=\"2\" align=\"right\"><input type=\"button\" onClick=\"history.go(-1);\" value=\"Back\" class=\"btnz btnz_color\"> <input type=\"submit\" name=\"submit\" value=\"Create Topic\" class=\"btnz btnz_color\"></td></tr>\n";
echo "</form></table>\n";
}else{
$cat = $_POST['cat'];
$title = $_POST['title'];
$msg = $_POST['message'];
if($cat && $title && $msg){
$sql = "SELECT level FROM sub_categories WHERE sub_id ='".$cat."'";
$res = mysql_query($sql) or die (mysql_error());
if(mysql_num_rows($res)==0){
echo "This forum sub category does not exist!\n";
} else{
$row = mysql_fetch_assoc($res);
if ($row['level'] == 1 && $admin_user_level !=1){
echo "You are not an admin therefore you cannot post a new topic!";
}else{
if (strlen($title) < 3 || strlen($title) > 1000){
echo "The title must between 3 and 1000 characters!\n";
}else{
if(strlen($msg) < 3 || strlen($msg) > 10000){
echo "The message must between 3 and 10,000 characters!\n";
}else{
$date = date("m-d-y") . " at " . date("h:i:s");
$time = time();
$sql2 = "INSERT INTO topics (topic_cid, topic_title, topic_uid, topic_date, topic_time, topic_message)
VALUES('".$cat."','".$title."','".$_SESSION['userid']."','".$date."','".$time."','".$msg."')";
$res2 = mysql_query($sql2) or die (mysql_error());
$tid = mysql_insert_id();
topic_go($tid);
}
}
}
}
}else{
echo "Please supply all fields!\n";
} }} }
}
}?>
Thank You For Helping!