first-image-to-post-thumbnail.php

<?php

	$args = array(
		'post_type'      => 'news',
		'posts_per_page' => -1,
	);
	$set_all_news_posts = get_posts( $args );
	foreach( $set_all_news_posts as $post ) {
		setup_postdata( $post );

		delete_post_thumbnail( $post );

		if ( preg_match_all( '/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches) ) {
			$image_url = $matches[1][0];
		}
		//var_dump( $image_url );

		// no thumbnail? on to the next
		if( empty( $image_url ) ) continue;

		// find mime type for later
		$filetype = wp_check_filetype( $image_url );

		if( has_post_thumbnail() ) {
			// Set up an array of args new attachment
			$args = array(
				'post_mime_type' => $filetype['type'],
				'post_title'     => esc_attr( $post->post_title ), // you may want something different here
				'post_content'   => '',
				'post_status'    => 'inherit'
			);

			// Insert the attachment!
			$thumb_id = wp_insert_attachment( $args, $image_url,  $post->ID );

			// gotta set up some meta data (height, width, etc)
			// must include this path
			require_once(ABSPATH . 'wp-admin/includes/image.php');
			$metadata = wp_generate_attachment_metadata( $thumb_id, $image_url );
			wp_update_attachment_metadata( $thumb_id, $metadata );

			// set post thumbnail :)
			update_post_meta( $post->ID, '_thumbnail_id', $thumb_id );
		}
	}
	wp_reset_postdata();