Useful

download_url( $target );
unzip_file( $file, $to );
wp_handle_sideload( $file );
media_sideload_image( $file, $post_id, $desc );

// For filters
__return_true();
__return_false();
__return_zero();
__return_empty_array();

// Serialization
maybe_serialize();
maybe_unserialize();
is_serialized();
is_serialized_string();

register_activation_hook( __FILE__, 'my_activation_hook' );

register_deactivation_hook( __FILE__, 'my_deactivation_hook' );

home_url(); // get_bloginfo('url');
site_url(); // get_bloginfo('wpurl');

dirname( __FILE__ );
plugins_url( 'a.php', __FILE__ );