Programmatically import images into WordPress with PHP

functions.php

<?php

    /**
    * Grab a remote image and import it into WordPress with PHP    
    */

    function mytheme_import_image($image_url){

        //$image_url is something like https://www.mysite.com/images/myimage.jpg

        //1. Get image
        $image_data = file_get_contents($image_url);

        //2. Build a filename
        $wordpress_upload_dir = wp_upload_dir();        
        
        $image_name = basename($image_url); // myimage.jpg
        $new_file_path = $wordpress_upload_dir['path'] . '/' . $image_name; // /new/path/to/image.jpg
        
        //if necessary, rename our image until it has a unique filename
        $i = 1;
        while( file_exists( $new_file_path ) ) {
            $i++;
            $new_file_path = $wordpress_upload_dir['path'] . '/' . $i . '_' . $image_name;
        }

        //save image locally
        file_put_contents($new_file_path, $image_data);

        if(!file_exists($new_file_path)) die("failed to write image locally"); //the image is not where we expect - check file permissions etc

        //3. Import into WordPress
        $new_file_mime = mime_content_type( $new_file_path );    
        
        $upload_id = wp_insert_attachment( array(
            'guid'           => $new_file_path,
            'post_mime_type' => $new_file_mime,
            'post_title'     => preg_replace( '/\.[^.]+$/', '', $image_name ),
            'post_content'   => '',
            'post_parent' => 0, //you can set this to a post ID if you want to associate this image with existing content
            'post_status'    => 'inherit'
        ), $new_file_path );
        
        if(is_wp_error($upload_id)) die($upload_id->get_error_message()); //something went awry
        
        wp_update_attachment_metadata( $upload_id, wp_generate_attachment_metadata( $upload_id, $new_file_path ) );
      
        //Now you can use $image_id however you want, eg update_post_meta($some_post_id, '_thumbnail_id', $upload_id) to set the featured image on $some_post_id
        
        return $upload_id;

    }//mytheme_import_image()

?>

Note if you are using this in a bootstrap WP script, you need to require_once('/webs/new/wp-admin/includes/media.php');

NB wp_generate_attachment_metadata seems to be in /wp-admin/includes/image.php (now?)