就我个人而言，我会创建hidden类型的<input>标签，并在其中存储PHP的search和secondary_variable参数，然后根据需要修改它们，并在论坛提交后进行搜索。
示例：
主文件：

<form action="submit.php" method="get">
    <input type="hidden" name="secondary_variable" value="<?php echo htmlspecialchars($_GET['secondary_variable']); ?>">
    <input type="text" name="search">
</form>

submit.php

<?php
    $search = htmlspecialchars($_GET['search']);
    $secondary_variable = htmlspecialchars($_GET['secondary_variable']);
    
    //// Do some PHP magic here, or just use the header() function to redirect the user to the search page
    header('Location: http://example.com/search?search='.$search.'&secondary_variable='.$secondary_variable);
?>