PHP string inside get_post_thumbnail_id

Hello,
This is my first time I'm using WMPU for help. I'm making a custom theme so I don't know if I'm in the right place.

I don't know if this a WordPress bug or not but my website doesn't recognize $post->ID. So I have a string that stores the post ID but I don't know how to execute that string inside wp_get_attachment_image_src?

The String: <?php $currentID = $post->ID;?>

The code: <?php echo wp_get_attachment_image_src( get_post_thumbnail_id('$currentID'), 'small-post' )[0]; ?>

Am I doing this right or is there another way?

Thanks,