avoiding duplicate entries and caching entered form values - php

Question

i have some code that controls duplicate entries in particular the USER ID. it checks in the database at submit and if that USER ID exists already it gives that notification. now the problem is when i submit and if that USER ID entered already exists in the database, all the other entries on the form are cleared, prompting me to re_enter all the other details again. i find this annoying and retrogressive. i want some help on how better i can do it such that only the USER ID text box returns empty, keeping other details safe/unchanged or indeed alternatively keeping/buffering/caching all details previously entered so that i can also review the duplicate USER ID before changing it.
new_user.php
<h1 align="center">Create New User</h1>
<p align="center" style="color:#F00"><?php if(isset($_GET['dup'])){ echo "That User ID Already Exists!"; } ?> </p>
<form id="form1" method="post" action="add_user.php">
<table width="100%">
<tr>
<td width="204"><div align="right">User ID:</div></td>
<td width="286">
<input type="text" name="user_id" id="user_id" />
</td>
</tr>
<tr>
<td><div align="right">Full Names:</div></td>
<td>
<input type="text" name="fname" id="fname" />
</td>
</tr>
<tr>
<td><div align="right">Gender:</div></td>
<td><select id="sex" name="sex">
<option selected="selected" value="male">Male</option>
<option name="female">Female</option>
</select></td>
</tr>
<tr>
<td><div align="right">NRC Number:</div></td>
<td>
<input type="number" name="nrcno" id="nrcno" min="1" />
</td>
</tr>
<tr>
<td><div align="right">Phone Number:</div></td>
<td>
<input type="number" name="cellno" id="cellno" />
</td>
</tr>
<tr>
<td><div align="right">Email Address:</div></td>
<td>
<input type="email" name="emailad" id="emailad" />
</td>
</tr>
<tr>
<td><div align="right">Position Held:</div></td>
<td>
<input type="text" name="posh" id="posh" />
</td>
</tr>
<tr>
<td> </td>
<td><input type="submit" name="create" id="create" value="Add User" /></td>
</tr>
</table>
</form>
add_user.php
<?php
$user_id=$_POST['user_id'];
$fname = $_POST['fname'];
$sex= $_POST['sex'];
$name= $_POST['name'];
$nrcno = $_POST['nrcno'];
$cellno= $_POST['cellno'];
$emailad = $_POST['emailad'];
$posh = $_POST['posh'];
require("get_func.php");
checkID($id);
include("connect.php");
mysql_select_db("ceec", $con);
$query = "INSERT INTO user VALUES ('$user_id', '$fname', '$sex','$name', '$nrcno', '$cellno', '$emailad', '$posh')";
if (mysql_query($query)){
header("Location: success.php");
}
else {echo "Nada" . mysql_error(); }
mysql_close($con);
?>
get_func.php
<?php
function checkID($id){
include_once("connect.php");
mysql_select_db("ceec",$con);
$query = "SELECT * FROM user WHERE user_id = '$id'";
$result= mysql_query($query);
if($row = mysql_fetch_array($result))
{
header("Location: new_user.php?dup=true");
break;
}
else {}
}
?>

Answer 1

<input type="text" name="user_id" id="user_id"
<?php if(isset($_POST['user_id'])){echo htmlentities($_POST['user_id'];} ?>/>