Getting “Object of class WP_Post could not be converted to string” - when it is a string

Question

I have the following code in my functions.php, which executes a script upon publish:

function save_new_post($post_ID) {

$site_root = '/home/forexmag/public_html/directory2';
$post = get_post($post_ID);
$title = $post->post_title;
$breaking_news = false;

$categories = get_the_category($post_ID);
if (is_array($categories) && !empty($categories))
{
    foreach ($categories as $cat_det)
    {
        //this is the id for the breaking (bad) news
        if (5668 == $cat_det->cat_ID)
        {
            $breaking_news = true;
            break;
        }
    }
}

$exec_code = "/usr/local/bin/php 
        $site_root 
        /crons/cron_notify.php '$title' $post_ID 2 " . intval($breaking_news);
exec(
    $exec_code
);
} add_action('draft_to_publish', 'save_new_post', 10, 1);

When I publish a new post I keep getting the following error:

Catchable fatal error: Object of class WP_Post could not be converted to string in /home/forexmag/public_html/wp-content/themes/forex_magnates/functions.php on line 712

This line is where I set the $exec_code var. However, as you can see, and as I confirmed via var_dump, the $title variable is a string. What am I missing?

EDIT: What's even weirder is that when I var_dump $exec_code and kill the script, I get a perfect string:

string(121) "/usr/local/bin/php /home/forexmag/public_html/directory2/crons/cron_notify.php '2011 Free Forex Report' 9934 2 0"

Answer 1

Got the correct answer on Wordpress Answers. Thought I should share it:

$post_ID is a WP_Post object. Your exec code should effectively be using $post_ID->ID.

function save_new_post($post_ID)
{
    print_r($post_ID);
    die();

Returns

WP_Post Object ( [ID] => ...

Courtesy of vancoder

Do notice that this depends on the hook you are using. E.g if you use publish_page you get an integer (This was actually the source of my confusion)