Get the first image in a post; retrieve the first image from each post in WordPress and resize it.

get-first-image-resize.php

/**
 * Get the first image from each post and resize it.
 *
 * @return string $first_img.
 * @link https://css-tricks.com/snippets/wordpress/get-the-first-image-from-a-post/
 * @link https://gist.github.com/SilentComics/0a7ea47942eb759dbb48eac2b7be1bbc/
 * Usage: Include in functions.php
 * Call <?php echo get_first_image('thumbnail'); ?> in page template.
 */
function get_first_image() {
	global $post;
	$first_img = '';
	preg_match_all( '/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', do_shortcode( $post->post_content, 'gallery' ), $matches );
	  $first_img = isset( $matches[1][0] ) ? $matches[1][0] : null;

	if ( empty( $first_img ) ) {
			return get_template_directory_uri() . '/assets/images/empty.png'; // path to default image.
	}

		// Now we have the $first_img but we want the thumbnail of that image.
		$explode = explode( '.', $first_img );
		$count = count( $explode );
		$size = '-624x312'; // Our panel ratio (2:1) 312x156 for lighther page, 624x312 for retina; use add_image_size() and Force Regenerate Thumbnails plugin when changing sizes.
		$explode[ $count -2 ] = $explode[ $count -2 ] . '' . $size;
		$thumb_img = implode( '.', $explode );
		return $thumb_img;
}
	add_filter( 'get_first_image', 'thumbnail' );

very thanks .
how can i force change the url of images that have another size !
like this :
https://yawar.ir/wp-content/uploads/2014/08/yawar-1024x682.jpg
in this case your code can't working correctly .

Until a better solution exists, you may want to use the Regenerate Thumbnails plugin when changing sizes.

Edit: Force Regenerate Thumbnails no longer seems maintained.