YA: Simple Form Processing AJDBNSOj3

<!-- task8.html -->
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
    <head>
        <title>Prac 3 Task 8</title>
    </head>
    <body>
        <h1>Customer Order Information</h1>
        <form id="userinfo" action="task8.php" method="post"><!-- Method switched to post, cleaner.-->
            <p>Search For A Customer</p>
            <p>
                Customer ID <input type="text" name="customer" />
            </p>
            <p>
                <input type="submit" value="submit"/>
                <input type="reset" value="reset"/>
            </p>
        </form>
    </body>
</html>

and my php:
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
    <head>
        <title>Tack 8.php</title>
    </head>
    <body>
<?php

if( isset($_POST['customer'])){

    // You should at least be using MySQLi Funtions instead of MySQL, as MySQL functions are deprecated.
    $conn = mysql_connect("localhost", "username", "password") or die( 'Cannot Connect to MySQL<br/>' . mysql_error());
    mysql_select_db("warehouse###", $conn) or die ('Database not found ' . mysql_error( $conn ) );

    $customer = mysql_real_escape_string( $_POST['customer'] ); // MySQL injection protection

    $query = "SELECT customerID FROM customer WHERE customerID='{$customer}' LIMIT 1";
    // No need to return all the data, if you are simply looking if it exists,
    // one column will do. If you later want to also display the info, change
    // the return column to appropriate column names or '*'

    $res = mysql_query($query) or die ("ERROR: sql = \"".$query."\"<br>" . mysql_error($conn) );
    if($res && mysql_num_rows($res) === 1){ // If the results are valid (although the die with the query makes this check already) and the number of returned rows is 1, then you have success.
        echo "Success";
    } else {
        echo "Fail";
    }
}else{
    echo '<h2 style="color: #A00;">Error</h2>';
    echo '<p>This page requires form input from here.</p>';
}
?>
</body>
</html>