Filter posts by post title first letter in WordPress (PHP 7.x)

filter-posts-by-post-title-first-letter.php

<?php

/**
 * Filter posts by post title first letter in WordPress (PHP 7.x)
 */
add_filter('posts_where', function (string $where, WP_Query $query): string {
    $wpdb = $GLOBALS['wpdb'];

    $starts_with = esc_sql($query->get('title_starts_with'));

    if (empty($starts_with)) {
        return $where;
    }

    $where .= $wpdb->prepare(" AND $wpdb->posts.post_title LIKE %s", $starts_with . '%');

    return $where;
}, 10, 2);

usage.php

<?php

$query = new WP_Query([
    'post_type' => 'post',
    'title_starts_with' => 'A',
]);

// OR

add_action('pre_get_posts', function (WP_Query $query): void {
    if (! $query->is_main_query() || ! $query->is_post_type_archive('post')) {
        return;
    }
    
    $query->set('title_starts_with', 'A');
}, 10, 1);