当select值包含多个单词时，如何基于先前的select选择更改select的值

// HTML
<select id="firstSelect">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
</select>

<select id="secondSelect">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
</select>

// JavaScript
const firstSelect = document.getElementById('firstSelect');
const secondSelect = document.getElementById('secondSelect');

firstSelect.addEventListener('change', function() {
  const selectedValue = firstSelect.value;
  
  // 根据先前的选择值修改后续select的选项
  if (selectedValue === 'option1') {
    secondSelect.innerHTML = `
      <option value="option3">Option 3</option>
      <option value="option4">Option 4</option>
    `;
  } else if (selectedValue === 'option2') {
    secondSelect.innerHTML = `
      <option value="option5">Option 5</option>
      <option value="option6">Option 6</option>
    `;
  }
});

<!-- HTML -->
<form method="post" action="process.php">
  <select name="firstSelect">
    <option value="option1">Option 1</option>
    <option value="option2">Option 2</option>
  </select>
  
  <select name="secondSelect">
    <?php
      // 处理先前select的选择值
      $selectedValue = $_POST['firstSelect'];
      
      // 根据先前的选择值生成后续select的选项
      if ($selectedValue === 'option1') {
        echo '
          <option value="option3">Option 3</option>
          <option value="option4">Option 4</option>
        ';
      } else if ($selectedValue === 'option2') {
        echo '
          <option value="option5">Option 5</option>
          <option value="option6">Option 6</option>
        ';
      }
    ?>
  </select>
  
  <input type="submit" value="Submit">
</form>

<!-- process.php -->
<?php
  // 处理表单提交的数据
  $selectedValue = $_POST['firstSelect'];
  
  // 根据先前的选择值生成后续select的选项
  if ($selectedValue === 'option1') {
    echo '
      <option value="option3">Option 3</option>
      <option value="option4">Option 4</option>
    ';
  } else if ($selectedValue === 'option2') {
    echo '
      <option value="option5">Option 5</option>
      <option value="option6">Option 6</option>
    ';
  }
?>