Plugin error: Use of undefined constant ABSPATH

My plugin (personalized for extension on my website) have this function:

function install_tables () {
   	global $wpdb;

   	$be = $wpdb->prefix . "browser_extension";

	$sql = "CREATE TABLE IF NOT EXISTS '".$be."' (
  'id' int(11) NOT NULL AUTO_INCREMENT,
  'date' datetime NOT NULL,
  'merchant' int(11) NULL,
  'code' varchar(64) NULL,
  'voucher_id' int(11) NULL,
  'voucher_nome' varchar(128) NULL,
  'url' varchar(256) NULL,
  'browser' varchar(64) NULL,
  PRIMARY KEY ('id')
) ENGINE=InnoDB DEFAULT CHARSET=latin1";

	require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
	$wpdb->query($sql);
}

register_activation_hook(__FILE__,'install_tables');
add_action("parse_request", "api");

and now when I try to activate it, shows me the following error in my Nginx log:

2013/07/22 17:21:54 [error] 19071#0: *276037 FastCGI sent in stderr: "PHP message: PHP Notice:  Use of undefined constant WP_CONTENT_DIR - assumed 'WP_CONTENT_DIR' in /srv/www/wp-admin/includes/upgra$
PHP message: PHP Notice:  Use of undefined constant ABSPATH - assumed 'ABSPATH' in /srv/www/wp-admin/includes/upgrade.php on line 16
PHP message: PHP Warning:  require_once(ABSPATHwp-admin/includes/admin.php): failed to open stream: No such file or directory in /srv/www/wp-admin/includes/upgrade.php on line 16
PHP message: PHP Fatal error:  require_once(): Failed opening required 'ABSPATHwp-admin/includes/admin.php' (include_path='.:/usr/share/php:/usr/share/pear') in /srv/www/wp-admin/includes/upgrade.php$

I observe that the error started after I updated the version of PHP 5.3.10 to 5.4.17. I do not want to downgrade. Anyone have another solution?