Exporting data in a WordPress widget

Question

I have some custom tables in a Wordpress database that I need to export to CSV. I created a dashboard widget to help with this based on some assistance given to me on on one of my SO questions. It basically consists of a single button that posts back and does some output buffering to render the CSV.

I've had it working correctly for a few weeks now, but when I went to push it live on our launch date, I ran into some odd behavior. The gist is that when I export locally on my dev box (Windows 7/Xampp) or on my personal hosted site (Linux) everything exports fine, but when I export on the client's server (Linux) the data is exported, but the CSV also contains the markup for the rest of the wp-admin dashboard page that follows after the widget is rendered. For example:

joe, blow
john, smith
<div><strong>Last entry: </strong>Hank Jones</div>
<div><strong>Total submissions: </strong> 7</div>
.. remaining markup removed for brevity

You can see that the CSV contains the <div><strong>Last entry: markup (and a whole lot more). I have no idea why this would be different based on different servers, but I wanted to post my plugin code in hopes that someone can see what's going on (and tips for improvement if applicable).

<?php
/*
Plugin Name: My Plugin
Description: Exporting Stuff
Author: tjans
Version: 1.0
Author URI: http://example.com
 */
add_action('init', 'myplugin_export_buffer_start');
add_action('in_admin_footer', 'myplugin_export_buffer_end');

function myplugin_export_ob_callback($buffer) {
    return $buffer;
}

function myplugin_export_buffer_start() 
{ 
    ob_start("myplugin_export_ob_callback"); 
}

function myplugin_export_buffer_end() 
{ 
    ob_end_flush(); 
}

function myplugin_display_mydata_export_widget_process()
{
    $errors = array();

    if ( 'POST' == $_SERVER['REQUEST_METHOD'] && isset ( $_POST['myplugin_export_button'] ))
    {
        ob_clean(); // erase the output buffer...blow away all the code up to this point

        global $wpdb;
        $tablePrefix = $wpdb->prefix;

        $qry = "QUERY TO PULL EXPORT DATA";

        $result = $wpdb->get_results($qry, ARRAY_A);
        if ($wpdb->num_rows)
        {
            $date = new DateTime();
            $ts = $date->format("Y-m-d-G-i-s");
            $filename = "mydata-export-$ts.csv";
            header( 'Content-Type: text/csv' );
            header( 'Content-Disposition: attachment;filename='.$filename);

            $fp = fopen('php://output', 'w');

            // show a header
            $headRow = $result[0];
            fputcsv($fp, array_keys($headRow));

            foreach ($result as $data) 
            {
                foreach($data as $field=>$value) 
                {
                    $data[$field] = stripslashes($value);
                }
                fputcsv($fp, $data);
            }
            fclose($fp);

            $contLength = ob_get_length();
            header( 'Content-Length: '.$contLength);
        }
    }

    return myplugin_display_mydata_export_form_widget();
}   

function myplugin_display_mydata_export_form_widget()
{
    global $wpdb;
    $tableName = $wpdb->prefix . "myplugin_mydata";

    $submissionCount = $wpdb->get_var("select count(Id) from $tableName");
    $lastUser = $wpdb->get_row("QUERY TO RETRIEVE LAST RECORD", ARRAY_A);
?>
    <?php if($lastUser) { ?><div><strong>Last entry: </strong><?php echo $lastUser['FirstName'] ?> <?php echo $lastUser['LastName'] ?> (<?php echo date('F jS, Y - g:i a', $lastUser['DateStamp']) ?>)</div><?php } ?>
    <div><strong>Total submissions: </strong> <?php echo $submissionCount ?></div>

    <form id="myplugin_mydata_export_widget" method="post" action="">
        <input type="submit" name="myplugin_export_button" value="Export All" />
    </form>
<?php
}

function myplugin_add_mydata_export_dashboard_widgets()
{
    // widget_id, widget_name, callback, control_callback
    wp_add_dashboard_widget(
        'myplugin-mydata-export-form-widget', 
        'Form Submissions', 
        'myplugin_display_mydata_export_widget_process'
    );  
}

add_action('wp_dashboard_setup', 'myplugin_add_mydata_export_dashboard_widgets' );

Answer 1

Normally, Output Buffer functions are not needed in a WordPress context, here neither. We'll just hook on admin_init to intercept our $_POST action, output the export file and exit. The browser doesn't even reloads the page.

The form needs a security field, with wp_nonce_field(), and a hidden one with our control action <input type="hidden" name="my_action" value="my_export" />.

The export code is based on a Pippin Plugins tutorial. He uses two functions I didn't know, ignore_user_abort() (PHP) and nocache_headers() (WP).

You're using myplugin as prefix, but it's better to personalize that, like tjs_ for example. But here I'll use a suffix, _CODEREVIEW-SE_QUESTION-ID (_crse_27734). Anyway, it's better to go OOP and drop this pre-suf-fixing.

I prefer to declare all actions first and then write the functions. I've stripped your data and replaced with simple/dummy info for replication purposes.

<?php
/**
 * Plugin Name: (CR) Export DB as CSV
 * Description: Exporting a database Query as CSV
 * Plugin URI: http://codereview.stackexchange.com/a/52051/14985
 * Author: brasofilo
 * Version: 1.0
 */

add_action( 'wp_dashboard_setup', 'add_widgets_crse_27734' );
add_action( 'admin_init', 'csv_export_crse_27734' );


/**
 * Create Dashboard Widget
 *
 * @return void
 **/
function add_widgets_crse_27734()
{
    wp_add_dashboard_widget(
        'export-form-widget', 
        'Form Submissions', 
        'display_widget_crse_27734'
    );  
}

/**
 * Print Dashboard Widget
 *
 * @return void
 **/
function display_widget_crse_27734()
{
    # Dummy data
    $submissionCount = 10;
    $lastUser = array('FirstName' => 'John', 'LastName' => 'Smith', 'DateStamp' => '12569537329' );
    ?>
    <?php if($lastUser) { 
        ?><div><strong>Last entry: </strong>
        <?php echo $lastUser['FirstName'] ?> 
        <?php echo $lastUser['LastName'] ?> 
        (<?php echo date('F jS, Y - g:i a', $lastUser['DateStamp']) ?>)</div>
    <?php } ?>
    <div><strong>Total submissions: </strong> <?php echo $submissionCount ?></div>
    <form method="post">
        <?php wp_nonce_field( 'export_nonce_crse_27734', 'export_nonce_crse_27734' ); ?>
        <input type="hidden" name="action_crse_27734" value="export_csv" />
        <input type="submit" name="myplugin_export_button_crse_27734" value="Export All" />
    </form>
    <?php
}


/**
 * Export CSV File
 *
 * @return void
 **/
function csv_export_crse_27734() 
{
    if( !isset( $_POST['action_crse_27734'] ) || 'export_csv' != $_POST['action_crse_27734'] )
        return;

    if( ! wp_verify_nonce( $_POST['export_nonce_crse_27734'], 'export_nonce_crse_27734' ) )
        return;

    if( ! current_user_can( 'manage_options' ) )
        return;

    $output = db_data_crse_27734();
    if( empty( $output ) )
        return;

    ignore_user_abort( true );
    nocache_headers();
    header( 'Content-Type: text/csv; charset=utf-8' );
    header( 'Content-Disposition: attachment; filename=code-review-export-' . date( 'Y-m-d-G-i-s' ) . '.csv' );
    header( "Expires: 0" );
    echo $output;
    exit;
}

/**
 * Query DB and build CSV string
 *
 * @return string
 **/
function db_data_crse_27734()
{
    global $wpdb;

    # Dummy query
    $args = array( 'ID', 'post_title', 'post_date', 'post_type' );
    $sql_select = implode( ', ', $args );
    $result = $wpdb->get_results("
            SELECT $sql_select
                FROM $wpdb->posts
                WHERE post_status = 'publish' 
    ");

    $output = '';
    if( $result )
    {
        foreach ( $result as $data ) 
        {
            $out_data = array();
            foreach( $data as $field => $value ) 
            {
                $out_data[$field] = stripslashes( $value );
            }
            $output .= implode( '|', $out_data ) . "\r\n";
        }
    }
    return $output;
}