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__ );