MySQL:(PHP) UPDATE Query issue - php

Question

I have an UPDATE Query in MySQL
UPDATE users
SET fname='$fname',lname='$lname',username='$user',password='$pass',role='$role',status=$status
where id=$id
It works perfectly when I run it in Phpmyadmin by giving values manually but it gives me an error message when I use it in PHP Page.
Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1
The error is generated because I am doing echo mysql_error() I am getting values from the FORM like this;
$id=$_POST['id'];
$fname=$_POST['txt_fname'];
$lname=$_POST['txt_lname'];
$user=$_POST['txt_user'];
$pass=$_POST['txt_pass'];
$role=$_POST['txt_role'];
$status=$_POST['status'];
Frankly I sound like a stupid asking such a simple question, but I am stuck and need a bit of help. Thanks
FORM HTML
<div id="cuntable" style=" width:400px; margin:0 auto;">
<div class="CSSTableGenerator">
<form name="userform" action="updateuser.php" method="post">
<table >
<tr>
<td colspan="2"> Edit / Modify User Details </td>
</tr>
<tr>
<td>First Name </td>
<td><input type="text" name="txt_fname" placeholder="<?php echo $row[1];?>" /></td>
</tr>
<tr>
<td>Last Name </td>
<td><input type="text" name="txt_lname" placeholder="<?php echo $row[2];?>" /></td>
</tr>
<tr>
<td>Username</td>
<td><input type="text" name="txt_user" placeholder="<?php echo $row[3];?>"/></td>
</tr>
<tr>
<td>Password</td>
<td><input type="password" name="txt_pass" placeholder="<?php echo $row[4];?>" /></td>
</tr>
<tr>
<td>Username</td>
<td>
<select name="txt_role">
<option value=""> Select User Role </option>
<option value="user"> User </option>
<option value="admin">Administrator</option>
<option value="operator">Operator</option>
<option value="accountant">Accountant </option>
</select>
</td>
</tr>
<tr>
<td> Status </td>
<td>
<select name="status">
<option value="1"> Active </option>
<option value="0"> Suspended </option>
</select>
</td>
</tr>
</table>
</div>
<br />
<input type="submit" value=" ADD USER " />
<input type="hidden" name="txt_id" value="<?php echo $row[0];?>"
</form>