Question

The following sample plugin lets you download a text file when the download button is pressed in the admin panel. The problem is that I expected the text file name would be hello_world.txt but it somehow becomes options-general.txt.

If this line header('Content-Disposition: attachment; filename="' . $file_name . '"'); is directly set the file name like header('Content-Disposition: attachment; filename="hello_world.txt"'); it works fine.

/* Plugin Name: Sample Download Button  */

$sample_download_button = new Sample_Download_Button_AdminPage('Sample Download Button');
add_action('init', array($sample_download_button, "download_text"));
add_action('admin_menu', array($sample_download_button , "admin_menu"));

class Sample_Download_Button_AdminPage {
    function __construct($pagetitle, $menutitle='', $privilege='manage_options', $pageslug='') {
        $this->pagetitle = $pagetitle;
        $this->menutitle = !empty($menutitle) ? $menutitle : $pagetitle;
        $this->privilege = $privilege;
        $this->pageslug = !empty($pageslug) ? $pageslug : basename(__FILE__, ".php");
    }
    function admin_menu() {
        add_options_page($this->pagetitle, $this->menutitle, $this->privilege, $this->pageslug, array(&$this, 'admin_page'));
    }
    function admin_page() {     
        ?>
        <div class="wrap">
            <h1>Download Button Sample</h1>
            <form action="" method="post" target="_blank">
            <input type="submit" value="download" name="download">
            </form>
        </div>
        <?php
    }   
    function download_text($file_name='hello_world.txt') {
        if (!isset($_POST['download'])) return;
        $your_text = 'hello world';
        header("Content-type: text/plain");
        header('Content-Disposition: attachment; filename="' . $file_name . '"');
        echo $your_text;
        exit;
    }       
}

Why is it? And how can I set a default parameter value? I tried it with a normal function and its default value is also not reflected. Thanks for your info.

Answer 1

I tested this a couple of ways, including copying your complete function into a ‘sandbox’ install I have.

Not all hooks pass parameters. From what I can tell, the init hook does not. Whether it accepts/passes a parameter is determined when the hook is defined. Apparently, when a hook fires that wasn’t intended to pass a parameter, that passes an empty string to the callback. In your case, this means that it is effectively overwriting your default and leaving you without a filename, which in turn causes the browser to use the file name of the page to which the form was submitted– options-general.php.

I’d pass the file name as a hidden field in the form and send it in $_POST, and set my default the way your have set $your_text.

function download_text() {
    if (!isset($_POST['download'])) return;
    if (!isset($_POST['filename'])) $file_name = $_POST['filename'];
    else $file_name = 'hello_world.txt';
    $your_text = 'hello world';
    header("Content-type: text/plain");
    header('Content-Disposition: attachment; filename="' . $file_name . '"');
    echo $your_text;
    exit;
}