Get posts group by alphabets and show it with shortcode.

get-posts-group-by-alphabet.php

<?php
/**
 * Get posts group by alphabets.
 * 
 * @todo Change the `prefix_` and with your own unique prefix.
 * 
 * @since 1.0.0
 */
if( ! function_exists( 'prefix_get_posts_group_by_alphabet' ) ) :
	function prefix_get_posts_group_by_alphabet() {
		$result = array();

		$query_args = array(
			'post_type'      => 'post',
		
			// Query performance optimization.
			'no_found_rows'  => true,
			'posts_per_page' => 100,
		);

		$query = new WP_Query( $query_args );
		if ( $query->posts ) {
			foreach ( $query->posts as $key => $post ) {
				$first_letter = substr($post->post_title,0,1);
				if( ! empty( $first_letter ) ) {
					$result[$first_letter][] = array(
						'ID' => $post->ID,
						'title' => $post->post_title,
					);
				}
			}
		}

		if( ! empty( $result ) ) {
			ksort( $result );
		}

		return $result;
   }
endif;