<?php
$username = 'myuser';
$plain_password = 'mypassword';
require_once('/path/to/wp-blog-header.php');
require_once('/path/to/class-phpass.php');
$userdata = get_user_by('login', $username);
$result = wp_check_password($plain_password, $userdata->user_pass, $userdata->ID);
if ( $result ) {
    echo "1";
} else {
    echo "0";
}
?>