Question

I want to make a simple form which displays error if a field has not been inputted. I don’t know how to do it. Here’s my code:
php code:

<?php

    //include the connection file

    require_once('connection.php');

    //save the data on the DB and send the email

    if(isset($_POST['action']) && $_POST['action'] == 'submitform')
    {
        //recieve the variables

        $name = $_POST['name'];
        $email = $_POST['email'];
        $message = $_POST['message'];
        $ip = gethostbyname($_SERVER['REMOTE_ADDR']);

        //save the data on the DB

        mysql_select_db($database_connection, $connection);

        $insert_query = sprintf("INSERT INTO feedback (name, email, message, date, ip) VALUES (%s, %s, %s, NOW(), %s)",
                                sanitize($name, "text"),
                                sanitize($email, "text"),
                                sanitize($message, "text"),
                                sanitize($ip, "text"));

        $result = mysql_query($insert_query, $connection) or die(mysql_error());

        if($result)
        {
            //send the email

            $to = "[e-mail]";
            $subject = "New message from the website";

            //headers and subject
            $headers  = "MIME-Version: 1.0\r\n";
            $headers .= "Content-type: text/html; charset=iso-8859-1\r\n";
            $headers .= "From: ".$name." <".$email.">\r\n";

            $body = "New contact<br />";
            $body .= "Name: ".$name."<br />";
            $body .= "Email: ".$email."<br />";
            $body .= "Message: ".$message."<br />";
            $body .= "IP: ".$ip."<br />";

            mail($to, $subject, $body, $headers);

            //ok message

            echo "Your message has been sent";
        }
    }

    function sanitize($value, $type) 
    {
      $value = (!get_magic_quotes_gpc()) ? addslashes($value) : $value;

      switch ($type) {
        case "text":
          $value = ($value != "") ? "'" . $value . "'" : "NULL";
          break;    
        case "long":
        case "int":
          $value = ($value != "") ? intval($value) : "NULL";
          break;
        case "double":
          $value = ($value != "") ? "'" . doubleval($value) . "'" : "NULL";
          break;
        case "date":
          $value = ($value != "") ? "'" . $value . "'" : "NULL";
          break;
      }

      return $value;
    }
    ?>

<form id="ContactForm" method="post" action="mail.php">
                            <div class="wrapper"><input class="input" name="name" id="name" type="text" value="Name:" onBlur="if(this.value=='') this.value='Name:'" onFocus="if(this.value =='Name:' ) this.value=''" ></div>
                            <div class="wrapper"><input class="input" name="email" id="email" type="text" value="E-mail:" onBlur="if(this.value=='') this.value='E-mail:'" onFocus="if(this.value =='E-mail:' ) this.value=''" ></div>
                            <div class="textarea_box"><textarea cols="1" rows="1" onBlur="if(this.value=='') this.value='Message:'" onFocus="if(this.value =='Message:' ) this.value=''" >Message:</textarea></div>
                            <input type="hidden" id="action" name="action" value="submitform" />
                            <input type="submit" class="button" id="submit" name="submit" value="Submit" /> <input type="reset" class="button" id="reset" name="reset" value="Reset" />
                        </form>

Answer 1

Before saving the information into the database check to see if each submitted value contains valid data. If not, put the field name into an array. Once validation is complete check to see if the array is empty or not. If it is empty, save the info into your database. If it is populated re-display the form, populated with the submitted data, and an easy-to-read notice of what errors they made so they know what to fix.

Some PHP functions to look into are: filter_var(), ctype_*, and empty()

FYI, you should consider migrating away from mysql_* functions since they will soon be going away.