Stopping users posting more than once

Question

Before posting my form I am checking the database to see if there are any previous posts from the user. If there are previous posts then the script will kick back a message saying you have already posted.

The problem is that what I am trying to achieve isn't working it all goes wrong after my else statement. It is also probable that there is an sql injection vulnerability too. Can you help??4

<?php

include '../login/dbc.php';
page_protect();

$customerid = $_SESSION['user_id'];

$checkid = "SELECT customerid FROM content WHERE customerid = $customerid";

if ($checkid = $customerid) {echo 'You cannot post any more entries, you have already created one';}

else

$sql="INSERT INTO content (customerid, weburl, title, description) VALUES
('$_POST[customerid]','$_POST[webaddress]','$_POST[pagetitle]','$_POST[pagedescription]')";

if (!mysql_query($sql))
  {
  die('Error: ' . mysql_error());
  }
echo "1 record added";

?>

Answer 1

To answer the second part of your question: yes, you're very vulnerable to SQL injection:

$sql="INSERT INTO content (customerid, ...) VALUES ('$_POST[customerid]', ...)";
                                                     ^

This article explains SQL Injection and how to avoid the vulnerability in PHP.

Answer 2

You are missing curly brackets {}:

<?php

if ($checkid == $customerid) {echo 'You cannot post any more entries, you have already created one';}

else
{

$sql="INSERT INTO content (customerid, weburl, title, description) VALUES
('$_POST[customerid]','$_POST[webaddress]','$_POST[pagetitle]','$_POST[pagedescription]')";

if (!mysql_query($sql))
  {
  die('Error: ' . mysql_error());
  }
echo "1 record added";
}

?>