This is a WordPress plugin that demonstrates how dynamic CSV data are downloaded using the export button with Admin Page Framework.

admin-page-framework-csv-download.php

<?php
/**
 * Plugin Name: Admin Page Framework - CSV Download
 * Description: Demonstrates how a CSV file is downloaded using the export button.
 * Version: 0.0.1
 * Author: Michael Uno
 */

namespace AdminPageFrameworkCSVDownload;

class Load {

    public function __construct() {
        add_action( 'plugins_loaded', array( $this, 'load' ) );
    }

    public function load() {
        if ( ! class_exists( 'AdminPageFramework' ) ) {
            return;
        }
        include( __DIR__ . '/AdminPage.php' );
        new AdminPage;
        include( __DIR__ . '/CSVGenerator.php' );
        new CSVGenerator;
    }

}
new Load;

AdminPage.php

<?php

namespace AdminPageFrameworkCSVDownload;

class AdminPage extends \AdminPageFramework {

    public function setUp() {

        $this->setRootMenuPage( 'Settings' );

        // Add the sub menus and the pages
        $this->addSubMenuItems(
            array(
                'title'     => 'CSV Download',        // the page and menu title
                'page_slug' => 'apf_csv_download',         // the page slug
            )
        );

        $this->addInPageTabs(
            'apf_csv_download',
            array(
                'tab_slug'  => 'sample',    // avoid hyphen(dash), dots, and white spaces
                'title'     => __( 'Sample', 'admin-page-framework-csv-download' ),
            ),
            array(
                'tab_slug'  => 'download',
                'title'     => __( 'Download', 'admin-page-framework-csv-download' ),
            )
        );

    }

    /**
     * @callback add_action() do_{page slug}_{tab slug}
     */
    public function do_apf_csv_download_sample() {
        echo "<h2>Sample</h2>"
            . "<p>This is a sample tab.</p>";
    }

    /**
     * @callback add_action() load_{page slug}_{tab slug}
     */
    public function load_apf_csv_download_download() {

        $this->addSettingFields(
            array(
                'field_id'          => 'export',
                'save'              => false,
                'type'              => 'export',
                'value'             => __( 'Download', 'admin-page-framework-csv-download' ),
                'description'       => __( 'Export data as CSV.', 'admin-page-framework-csv-download' ),
                'file_name'         => 'test.csv',
                'show_title_column' => false,
            )
        );

    }

}

CSVGenerator.php

<?php

namespace AdminPageFrameworkCSVDownload;

class CSVGenerator {

    /**
     * Sets up properties and hooks.
     */
    public function __construct() {

        $_sPageSlug = 'apf_csv_download';
        $_sTabSlug  = 'download';
        add_filter( "export_{$_sPageSlug}_{$_sTabSlug}", array( $this, 'replyToFilterExportData' ), 10, 0 );
        add_filter( "export_name_{$_sPageSlug}_{$_sTabSlug}", array( $this, 'replyToGetExportFileName' ), 10, 0 );
        add_filter( "export_format_{$_sPageSlug}_{$_sTabSlug}", array( $this, 'replyToGetFormat' ), 10, 0 );

    }

    /**
     * @return string Force the output to be text.
     */
    public function replyToGetFormat() {
        return 'text';
    }

    /**
     * @callback add_filter() export_{page slug}_{tab slug}
     * @return   string
     */
    public function replyToFilterExportData() {
        // Simulate some data
        $_aData = array (
            array( 'Year', 'Make', 'Model', 'Description', 'Price' ),
            array( 1997, 'Ford', 'E350', 'ac, abs, moon', '3000.00' ),
            array( 1999, 'Chevy', 'E350', 'Venture "Extended Edition"', '4900.00' ),
            array( 1999, 'Chevy', 'E350', 'Venture "Extended Edition, Very Large"', '5000.00' ),
            array( 1996, 'Jeep', 'Grand Cherokee', 'MUST SELL! air, moon roof, loaded', '4799.00' ),
        );
        return $this->getCSVFromList( $_aData );
    }

    public function getCSVFromList( array $aList, $sDelimiter=',', $sEnclosure='"', $sEscapeChar= "\\" ) {
        $_rFile = fopen('php://memory', 'r+' ); // r+
        foreach( $aList as $_aFields ) {
            fputcsv( $_rFile, $_aFields, $sDelimiter, $sEnclosure, $sEscapeChar );
        }
        rewind( $_rFile );
        $_bsCSV = stream_get_contents( $_rFile );
        return false === $_bsCSV
            ? ''
            : rtrim( $_bsCSV );
    }

    /**
     * @return string
     */
    public function replyToGetExportFileName() {
        $_sToday = date( 'Ymd', time() );
        return "apf-csv-download-{$_sToday}.csv";
    }

}