Some useful functions to paste in your child theme for debugging

Display variable content :

function outpre(&$var) {
 if(empty($var)) {
 $output = "variable has no data";
 } else {
 echo "<pre style='background:white;color:navy;margin:5px;padding:10px;border-radius:5px;'>";
 print_r($var);
 echo "</pre>";
 }
}

Display all functions linked to a specific hook (uses outpre)

function show_hooked(){
    $hook_name = 'your_hook_here';
    global $wp_filter;
    outpre( $wp_filter[$hook_name] );
}
//add_action('wp_footer', 'show_hooked');

Display all queries fired to generate the page (uses outpre)

function get_db_queries()
{
    $a = 1;
    global $wpdb;
    foreach ($wpdb->queries as $query) {
        echo '<h3>QUERY N°'.$a.'</h3>';
        echo '<h4>Query</h4>';
        outpre($query);
        echo '<h4>Result</h4>';
        $result = $wpdb->get_results($query[0]);
        outpre($result);
        $a++;
    }
    wp_cache_flush();
}
//add_action( 'wp_footer', 'get_db_queries' );

More details and comments coming soon ! (hopefully…)