Email When Plugin or Theme Updates Are Available

add_action( 'init', function () {
    if ( ! wp_next_scheduled( 'wpide_updates_alert' ) ) {
        wp_schedule_event( time() + HOUR_IN_SECONDS, 'daily', 'wpide_updates_alert' );
    }
} );

add_action( 'wpide_updates_alert', function () {
    $lines   = array();
    $plugins = get_site_transient( 'update_plugins' );
    $themes  = get_site_transient( 'update_themes' );
    $core    = get_site_transient( 'update_core' );

    if ( ! empty( $plugins->response ) ) {
        if ( ! function_exists( 'get_plugins' ) ) {
            require_once ABSPATH . 'wp-admin/includes/plugin.php';
        }
        $installed = get_plugins();
        foreach ( $plugins->response as $file => $data ) {
            $name    = isset( $installed[ $file ]['Name'] ) ? $installed[ $file ]['Name'] : $file;
            $lines[] = sprintf( 'Plugin: %s → %s', $name, $data->new_version );
        }
    }
    if ( ! empty( $themes->response ) ) {
        foreach ( $themes->response as $slug => $data ) {
            $lines[] = sprintf( 'Theme: %s → %s', $slug, $data['new_version'] );
        }
    }
    if ( ! empty( $core->updates[0] ) && 'upgrade' === $core->updates[0]->response ) {
        $lines[] = 'WordPress → ' . $core->updates[0]->current;
    }

    $hash = md5( implode( '|', $lines ) );
    if ( ! $lines || get_option( 'wpide_updates_alert_hash' ) === $hash ) {
        return;
    }
    update_option( 'wpide_updates_alert_hash', $hash, false );

    wp_mail( get_option( 'admin_email' ), sprintf( '[%s] %d update(s) available', wp_specialchars_decode( get_bloginfo( 'name' ), ENT_QUOTES ), count( $lines ) ), "These updates are ready to install:\n\n" . implode( "\n", $lines ) . "\n\nUpdate now: " . admin_url( 'update-core.php' ) );
} );