Snippet to allow numeric URLs in WordPress

allow-numeric-urls.php

<?php

add_filter( 'wp_unique_post_slug', 'numeric_post_slug', 10, 6 );
function numeric_post_slug( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
    global $wpdb;

    if ( ! is_numeric( $original_slug ) || $slug === $original_slug ) {
        return $slug;
    }

    $post_name_check = $wpdb->get_var( $wpdb->prepare(
        "SELECT post_name FROM $wpdb->posts WHERE post_name = %s AND post_type IN ( %s, 'attachment' ) AND ID != %d AND post_parent = %d LIMIT 1",
        $original_slug, $post_type, $post_ID, $post_parent
    ) );

    if ( $post_name_check ) {
        return $slug;
    }

    return $original_slug;
}

?>