Validation means checking the input submitted by the user. Example below shows the form with required field validation

You will see how to use PHP to verify the different fields of forms so that after the user submits the form, even if the information provided is not valid

Example

<html>
    <head>
        <style>
            .error {color: #FF0000;}
        </style>
    </head>

    <body>
        <?php
        // define variables and set to empty values
        $nameErr = $emailErr = $genderErr = $websiteErr = "";
        $name = $email = $gender = $comment = $website = "";

        if ($_SERVER["REQUEST_METHOD"] == "POST") {
            if (empty($_POST["name"])) {
                $nameErr = "Name is required";
            } else {
                $name = test_input($_POST["name"]);
            }

            if (empty($_POST["email"])) {
                $emailErr = "Email is required";
            } else {
                $email = test_input($_POST["email"]);

                // check if e-mail address is well-formed
                if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
                    $emailErr = "Invalid email format";
                }
            }

            if (empty($_POST["website"])) {
                $website = "";
            } else {
                $website = test_input($_POST["website"]);
            }

            if (empty($_POST["comment"])) {
                $comment = "";
            } else {
                $comment = test_input($_POST["comment"]);
            }

            if (empty($_POST["gender"])) {
                $genderErr = "Gender is required";
            } else {
                $gender = test_input($_POST["gender"]);
            }
        }

        function test_input($data) {
            $data = trim($data);
            $data = stripslashes($data);
            $data = htmlspecialchars($data);
            return $data;
        }
        ?>

        <h2>Add Person to PHP Tutorial</h2>
        <p><span class = "error">* required field.</span></p>
        <form method = "post" action = "<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>">
            <table>
                <tr>
                    <td>Name:</td>
                    <td><input type = "text" name = "name">
                        <span class = "error">* <?php echo $nameErr; ?></span>
                    </td>
                </tr>
                <tr>
                    <td>E-mail: </td>
                    <td><input type = "text" name = "email">
                        <span class = "error">* <?php echo $emailErr; ?></span>
                    </td>
                </tr>
                <tr>
                    <td>Time:</td>
                    <td> <input type = "text" name = "website">
                        <span class = "error"><?php echo $websiteErr; ?></span>
                    </td>
                </tr>
                <tr>
                    <td>Classes:</td>
                    <td> <textarea name = "comment" rows = "5" cols = "40"></textarea></td>
                </tr>
                <tr>
                    <td>Gender:</td>
                    <td>
                        <input type = "radio" name = "gender" value = "female">Female
                        <input type = "radio" name = "gender" value = "male">Male
                        <span class = "error">* <?php echo $genderErr; ?></span>
                    </td>
                </tr>
                <td>
                    <input type = "submit" name = "submit" value = "Submit"> 
                </td>
            </table>
        </form>
        <?php
        echo "Your given values are as:";
        echo $name;
        echo $email;
        echo $website;
        echo $comment;
        echo $gender;
        ?>
    </body>
</html>



$_SERVER["REQUEST_METHOD"] - it contains a request (surprise). $ _POST contains any post data. then we are validating the name, email, and website URL giving by the User while submitting the form.