Testimonial Widget for question on WordPress Stack Exchange

testimonial_widget.php

<?php
// don't include opening php tag unless necessary

function testimonial_widget($args, $instance) {
  global $post;

  $current_page_id =  $post->ID;
  $current_page_id_string = strval($post->ID);

  // This queries for post_type of testimonal with a 
  // meta-select value of the current page ID

  // I think you may be tripping up because $post->ID is an integer
  // but the meta-select value you are saving is a string, so I am
  // going to try both here. 

  $query_args = array (
        'post_type'   => 'testimonial',
        'meta_query'  => array(
              array(
              'key'       => 'meta-select',
              'value'     => array( $current_page_id, $current_page_id_string ),
              'compare'   => 'IN',
              ),
        ),
  );

  $t_q = new WP_Query( $query_args );

  $title = apply_filters( 'widget_title', $instance['title'] );

  do_action('before_widget');
  
  $output = "<div id='testimonial-rotator' class='widget'>";
  $output .= "<h3 class='widgettitle'>" . $instance['title'] . "</h3> <ul>";

  if ( $t_q->have_posts() ) : 
    while ( $t_q->have_posts() ) : $t_q->the_post();  
    
      $output .= "<li id='testimonial-".the_ID()."'>";
      $output .= "<blockquote>".the_excerpt()."</blockquote>";
      $output .= "<p>".the_author()."</p>";
      $output .= "</li>";

    endwhile;
  endif; 

  $output .= "</ul> </div>";

  echo $output;

  do_action('after_widget');
  wp_reset_postdata();
}