<?php
$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 );
	}
}