Get the first image of post content.

get_first_image_of_content.php

<?php
/**
	 *  Get the first image of post content by post_id
	 */
	function catch_the_first_image($post_id)
	{
		$first_img = '';
		$content_post = get_post($post_id);
		$content = $content_post->post_content;
		$output = preg_match_all('/<img.+?src=[\'"]([^\'"]+)[\'"].*?>/i', $content, $matches);
		$first_img = $matches[1][0];

		if (empty($first_img)) {
			$first_img = get_stylesheet_directory_uri() . '/img/imageplaceholder.png';
		}
		return $first_img;
	}