WordPress shortcode to access external database and output data in HTML table.

wp-ext-db-html-table.php

<?php
/**
 * Shortcode accesses external database and returns data in HTML table.
 *
 * Shortcode is: [my-db-data]. 
 * Uses WP class:
 * @link https://developer.wordpress.org/reference/classes/wpdb/
 * Sets transient with db data and 24 hour expiration.
 *
 * @return string    $data_html    HTML table with database data
 */
function my_db_data_table() {
    $db_html = '';

    // Get any existing copy of our transient data
    if ( false === ( $data = get_transient( 'my_data_transient' ) ) ) {
        /* No transient, so access database and make new transient (uses wpdb class) */
        $db_data = new wpdb(
            'my_db_username',
            'my_db_password',
            'my_db_name',
            'my_db_host'
        );
        $data = $data_db->get_results( 'SELECT * FROM `my_db_table_name`.`my_db_column_name`', ARRAY_A );
        set_transient( 'my_data_transient', $data, DAY_IN_SECONDS );
    }

    /* Build HTML table */
    $data_html .= "<table class=\"table table-bordered table-hover info\"><thead>\n";
    $data_html .= "<colgroup>\n<col /><col /><col /><col /><col />\n</colgroup/>\n"; // For 5 cols.
    $data_html .= "<thead>\n<tr>\n";

    /* Build HTML table head (column names) from database keys */
    foreach ( $data[0] as $key => $value ) {
        $data_html .= "<th scope=\"col\">{$key}</th>\n";
    }

    /* Build HTML table cells from database values */
    foreach ( $data as $row ) {
        $data_html .= "<tr>\n";
        foreach ( $row as $key => $value ) {
            $data_html .= "<td>{$value}</td>\n";
        }
        $data_html .= "</tr>\n";
    }
    $data_html .= "</tbody>\n</table>\n";

    return $data_html;
}
add_shortcode('my-db-data', 'my_db_data_table');
?>