form_validasi.php

<?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"]);

        // check if name only contains letters and whitespace
        if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
            $nameErr = "Only letters and white space allowed";
        }
    }

    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"]);
        // check if URL address syntax is valid (this regular expression also allows dashes in the URL)
        if (!preg_match("/\b(?:(?:https?|ftp):\/\/|www\.)[-a-z0-9+&@#\/%?=~_|!:,.;]*[-a-z0-9+&@#\/%=~_|]/i",$website)) {
            $websiteErr = "Invalid URL";
        }
    }

    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;
}
?>

<!DOCTYPE HTML>
<html>
<head>
    <title>Form dan Validasi</title>
    <style>
        .error {color: #FF0000;}
    </style>
</head>
<body>

    <h2>PHP Form Validation Example</h2>

    <p><span class="error">* required field</span></p>

    <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">

    Name: <input type="text" name="name" value="<?php echo $name;?>">
    <span class="error">* <?php echo $nameErr;?></span>
    <br><br>

    E-mail: <input type="text" name="email" value="<?php echo $email;?>">
    <span class="error">* <?php echo $emailErr;?></span>
    <br><br>

    Website: <input type="text" name="website" value="<?php echo $website;?>">
    <span class="error"><?php echo $websiteErr;?></span>
    <br><br>

    Comment: <textarea name="comment" rows="5" cols="40"><?php echo $comment;?></textarea>
    <br><br>

    Gender:
    <input type="radio" name="gender" <?php if (isset($gender) && $gender=="female") echo "checked";?> value="female">Female
    <input type="radio" name="gender" <?php if (isset($gender) && $gender=="male") echo "checked";?> value="male">Male
    <input type="radio" name="gender" <?php if (isset($gender) && $gender=="other") echo "checked";?> value="other">Other
    <span class="error">* <?php echo $genderErr;?></span>
    <br><br>

    <input type="submit" name="submit" value="Submit">

    </form>

<?php
echo "<h2>Your Input:</h2>";
echo $name;
echo "<br>";
echo $email;
echo "<br>";
echo $website;
echo "<br>";
echo $comment;
echo "<br>";
echo $gender;
?>

</body>
</html>