Get WordPress attachment from post

get_random_attachment_id_from_post.php

<?php

function get_random_attachment_id_from_post($post = 0) {
	$post = get_post( $post );

	if ( ! preg_match_all( '/<img [^>]+>/', $post->post_content, $matches ) ) {
		return 0;
	}

	$attachment_ids = array();
	foreach( $matches[0] as $image ) {
		if ( preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) && ( $attachment_id = absint( $class_id[1] ) ) ) {
			$attachment_ids[] = $attachment_id;
		}
	}

	if ($attachment_ids) {
	    return $attachment_ids[0];
    }

    return 0;
}