import-image.php

<?php
function import_image($post_id, $image_url,$image_name){

	// Add Featured Image to Post
	//$image_url        = 'http://s.wordpress.org/style/images/wp-header-logo.png'; // Define the image URL here
	//$image_name       = 'wp-header-logo.png';
	$upload_dir       = wp_upload_dir(); // Set upload folder
	$image_data       = file_get_contents($image_url); // Get image data
	$unique_file_name = wp_unique_filename( $upload_dir['path'], $image_name ); // Generate unique name
	$filename         = basename( $unique_file_name ); // Create image file name

	// Check folder permission and define file location
	if( wp_mkdir_p( $upload_dir['path'] ) ) {
		$file = $upload_dir['path'] . '/' . $filename;
	} else {
		$file = $upload_dir['basedir'] . '/' . $filename;
	}

	// Create the image  file on the server
	file_put_contents( $file, $image_data );

	// Check image file type
	$wp_filetype = wp_check_filetype( $filename, null );

	// Set attachment data
	$attachment = array(
		'post_mime_type' => $wp_filetype['type'],
		'post_title'     => sanitize_file_name( $filename ),
		'post_content'   => '',
		'post_status'    => 'inherit'
	);

	// Create the attachment
	$attach_id = wp_insert_attachment( $attachment, $file, $post_id );

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

	// Define attachment metadata
	$attach_data = wp_generate_attachment_metadata( $attach_id, $file );

	// Assign metadata to attachment
	wp_update_attachment_metadata( $attach_id, $attach_data );

	// And finally assign featured image to post
	set_post_thumbnail( $post_id, $attach_id );
}
?>