Question

I am trying to redirect users to a custom error page (for example: error.php) when a duplicate entry is made, this is a sample of the insert statement, but I don’t know how to do the redirecting.

$values = $_POST;
foreach ($values as &$value) {
    $value = mysql_real_escape_string($value);
}

$sql1="INSERT INTO loan (loan_id)
VALUES ('$values[loan_id]')";

$result = mysql_query($sql1);
if (!$result) {
    die('Invalid query: ' . mysql_error());
}

Answer 1

The proper way to redirect form input is using the HTTP 303 redirect.

To redirect at the current line, do so something like this, in PHP:

// Tell the browser to redirect
header('Location: /error.php', TRUE, 303);

// This ensures that the script doesn't continue
// Also, it shows the error on the off chance their browser doesn't redirect
die('Input error.');

You must ensure that you include the header function before any output.

Here it is in your code:

if (!$result) {
    header('Location: /error.php', TRUE, 303);
    die('Invalid query: ' . mysql_error());
} else {
    header('Location: /success.php', TRUE, 303);
    die('Success');
}