Add file to media library programmatically

$file = '/path/to/file.png';

$filename = basename($file);

$upload_file = wp_upload_bits($filename, null, file_get_contents($file));

if (!$upload_file['error']) {

	$wp_filetype = wp_check_filetype($filename, null );

	$attachment = array(

		'post_mime_type' => $wp_filetype['type'],

		'post_parent' => $parent_post_id,

		'post_title' => preg_replace('/\.[^.]+$/', '', $filename),

		'post_content' => '',

		'post_status' => 'inherit'

	);

	$attachment_id = wp_insert_attachment( $attachment, $upload_file['file'], $parent_post_id );

	if (!is_wp_error($attachment_id)) {

		require_once(ABSPATH . "wp-admin" . '/includes/image.php');

		$attachment_data = wp_generate_attachment_metadata( $attachment_id, $upload_file['file'] );

		wp_update_attachment_metadata( $attachment_id,  $attachment_data );

	}

}