为什么我的PHP代码输入了“0”，而没有输入其他字段？

<form method="post" action="your_script.php">
    <input type="text" name="field1">
    <input type="text" name="field2">
    <input type="submit" value="Submit">
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $field1 = $_POST['field1'] ?? '';
    $field2 = $_POST['field2'] ?? '';

    echo "Field 1: " . htmlspecialchars($field1) . "<br>";
    echo "Field 2: " . htmlspecialchars($field2) . "<br>";
}
?>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $field1 = isset($_POST['field1']) ? $_POST['field1'] : 'default_value';
    $field2 = isset($_POST['field2']) ? $_POST['field2'] : 'default_value';

    echo "Field 1: " . htmlspecialchars($field1) . "<br>";
    echo "Field 2: " . htmlspecialchars($field2) . "<br>";
}
?>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $field1 = $_POST['field1'] ?? '';
    $field2 = $_POST['field2'] ?? '';

    // 假设使用MySQLi连接数据库
    $conn = new mysqli("localhost", "username", "password", "database");
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }

    $sql = "INSERT INTO your_table (field1, field2) VALUES (?, ?)";
    $stmt = $conn->prepare($sql);
    $stmt->bind_param("ss", $field1, $field2);
    $stmt->execute();

    echo "New record created successfully";
    $stmt->close();
    $conn->close();
}
?>