session_start() causes fatal error

I'll keep this simple. Why calling session_start() at the top of my PHP script, I'm getting this output at the bottom:

Fatal error: Exception thrown without a stack frame in Unknown on line 0

What's going on?

EDIT: Some further details as I've isolated the problem further.

index.php (excerpt):

<?php
session_start();

require_once('inc/database.php');
require_once('inc/gallery.php');
...

database.php:

<?php
try {
    $dsn = 'mysql:host=localhost;dbname=tees_db';
    $pdo = new PDO($dsn, '[username removed]', '[password removed]');
}
catch (PDOException $e) {
    header('HTTP/1.1 503 Service Unavailable');
    die('There was an error connecting to the database.');
}

gallery.php (excerpt):

<?php
class Gallery {
    private $pdo;
    public function __construct() {
        global $args, $pdo, $request;
        $this->pdo = $pdo;
    }
    ...
}
$gallery = new Gallery();

The problem comes in the Gallery class when I try and assign the global $pdo variable as a class property. Why would this cause a fatal error when $pdo is just a PDO instance?

EDIT 2: I've found closing my browser and re-launching (killing the session) suppresses the error. The error is only triggered when inc/confirm.php is called.

inc/confirm.php is a script used in over-18 confirmation. The contents are minimal:

session_start();

if (isset($_GET['mo开发者_开发百科d'])) {
    $mod = $_GET['mod'];
    $_SESSION[$mod] = '1';
}

header('Location: '.$_SERVER['HTTP_REFERER']);
exit;

As you can see, simply saves a key in the $_SESSION array and redirects back to the original page. Nothing about exceptions or class de-constructors in there.

I assume there is no echo or printing before session _start.

If there is any blank space before the function call,try to remove it.

Try ob_clean before calling session_start . i dont know the side effects. :(

Try

NO GLOBALS!!!!!!!

create a wrapper for your database connection (singleton class is 'OK' if you only have one db - if you have more then a modified one is required(holla if you need example)).

then just use

class Gallery {
    private $pdo;
    public function __construct() {

        $this->pdo = DBObj::getInst();
    }
    ...
}
$gallery = new Gallery();

if you can be bothered to refactor and still have probs just holla.

I don't actually think that is the problem here though...

I ran

session_start();

try {
    $dsn = 'mysql:host=localhost;dbname=DB';
    $pdo = new PDO($dsn, 'UN', 'PW');
}
catch (PDOException $e) {
    header('HTTP/1.1 503 Service Unavailable');
    die('There was an error connecting to the database.');
}

class Gallery {
    private $pdo;
    public function __construct() {
        global $args, $pdo, $request;
        $this->pdo = $pdo;
    }

}

$gallery = new Gallery();

and get no errors...