PHP: How to call a stored procedure

The following code snippet was created in MySQL Workbench:

DELIMITER //
CREATE PROCEDURE getCustomers()
BEGIN
	SELECT * FROM customers;
END //
DELIMITER ;

CALL getCustomers(); // This works just fine, I get the result set

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But I'm having a problem on the PHP side. Here's the code in PHP:

    $stmt = $db->prepare("call getCustomers()");
    $result = $stmt->execute();
    while ($row = $result->fetch(PDO::FETCH_ASSOC)) {
        echo $row['custid'] . '<br/>';
    }

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I am getting a PHP error that says, "Fatal error: Call to a member function fetch() on boolean in C:\xampp\htdocs\newdimension\public\DBPrep.php".

Can someone please tell me what I'm doing wrong? Thanks.