if ( substr( $key, - strlen( $method ) ) !== $method ) {

if ( substr( (string) $key, - strlen( $method ) ) !== $method ) {

function taktheme_preferred_source_script() {
    wp_enqueue_script(
        'google-preferred-source',
        'https://news.google.com/swg/js/v1/publisher.js',
        array(),
        null,
        true
    );
}
add_action('wp_enqueue_scripts', 'taktheme_preferred_source_script');

function taktheme_preferred_source_shortcode() {
    return '<div google-add-preferred-source-btn></div>';
}
add_shortcode('preferred_source', 'taktheme_preferred_source_shortcode');

function taktheme_preferred_source_content($content) {
    if (!is_singular(array('post', 'product'))) {
        return $content;
    }

    if (!in_the_loop() || !is_main_query()) {
        return $content;
    }

    $button = do_shortcode('[preferred_source]');

    return $button . $content;
}

add_filter('the_content', 'taktheme_preferred_source_content');