通过PHP提交表单后，保持下拉框的选中状态

<form method="post" action="submit.php">
    <select name="dropdown">
        <option value="option1">Option 1</option>
        <option value="option2">Option 2</option>
        <option value="option3">Option 3</option>
    </select>
    <input type="submit" value="Submit">
</form>

<?php
if($_SERVER["REQUEST_METHOD"] == "POST") {
    $selectedOption = $_POST["dropdown"];
}
?>

<form method="post" action="submit.php">
    <select name="dropdown">
        <option value="option1" <?php if($selectedOption == "option1") echo "selected"; ?>>Option 1</option>
        <option value="option2" <?php if($selectedOption == "option2") echo "selected"; ?>>Option 2</option>
        <option value="option3" <?php if($selectedOption == "option3") echo "selected"; ?>>Option 3</option>
    </select>
    <input type="submit" value="Submit">
</form>