如何使用PHP获取所选组合框项目的*idex*,而不是其文本？

<form action="process.php" method="post">
   <select name="combobox">
       <option value="1">选项1</option>
       <option value="2">选项2</option>
       <option value="3">选项3</option>
    </select>
   <input type="submit" value="提交">
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $selectedIndex = $_POST["combobox"];
    echo "您选择的项目的*index*是：" . $selectedIndex;
}
?>