Uncaught Error: Call to undefined function file_is_valid_image()

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I’m running the code above as WP cron

The file_is_valid_image function is defined in wp-admin/includes/image.php, and this file is loaded automatically only on WordPress admin pages (in wp-admin).

So if you want to use that function on other/non-admin pages, e.g. a page in the site’s front-end, a REST API endpoint, or wp-cron.php which runs during a WP-Cron request, you need to manually load wp-admin/includes/image.php, like so:

// load the file which defines the file_is_valid_image function
require_once ABSPATH . 'wp-admin/includes/image.php';

// now you can call the function, e.g. in your case:
if ( file_is_valid_image( $upload_basedir . "https://wordpress.stackexchange.com/" . $image_file ) ) ...

But actually, you don’t have to use file_is_valid_image(), because you can easily do what it does where it uses wp_getimagesize() which is loaded everywhere in WordPress:

// all you need to do is, replace this:
if ( file_is_valid_image( $upload_basedir . "https://wordpress.stackexchange.com/" . $image_file ) )

// with this:
$size = wp_getimagesize( $upload_basedir . "https://wordpress.stackexchange.com/" . $image_file );
if ( ! empty( $size ) )

References