Enable Simple Lightbox in feature image when on singular page.

enable-lightbox.php

/**
 * Enable lightbox in feature image when on singular page. 
 *
 * @see: https://wordpress.org/support/topic/help-activating-slb-in-my-theme?replies=14
 */
function toivo_post_thumbnail() {
	if ( post_password_required() || is_attachment() || ! has_post_thumbnail() ) {
		return;
	}

	if ( is_singular() && !is_page_template( 'pages/front-page.php' ) && !is_page_template( 'pages/child-pages.php' ) ) :
	
		if ( has_post_thumbnail() ) :
			$full_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'post-thumbnail' );
			$content = '<a class="post-thumbnail" href="' . $full_image_url[0] . '">' . get_the_post_thumbnail( $post_id, 'post-thumbnail' ) . '</a>';
			if ( function_exists('slb_activate') ) :
				$content = slb_activate( $content );
				echo $content;
			endif;
		endif;

	else : ?>

		<a class="post-thumbnail" href="<?php the_permalink(); ?>" aria-hidden="true">
			<?php
				the_post_thumbnail( 'post-thumbnail', array( 'alt' => get_the_title() ) );
			?>
		</a>

	<?php endif; // End is_singular()
}