Looks like you are using a deprecated version of PHP.
You should upgrade at least to PHP 5.3. Older versions of PHP are no longer supported by the PHP team itself.
See https://de3.php.net/downloads.php (current PHP downloads) and https://de3.php.net/releases/ (PHP 5.2 reached its end of life status at 6 Jan 2011).
Also the latest versions of Piwik (since 2.0) require PHP 5.3.2 or greater, see https://piwik.org/docs/requirements/
WP-Piwik requires Piwik 2.0.2 or higher since 0.9.9.7 (release 4 months ago).
As a workaround until you are able to (or your hoster is up to) update PHP, you can comment the callPHP function in wp-piwik.php:
Find line 741:
function callPHP($strParams) {
Change to:
function callPHP($strParams) { /*
Find line 757:
}
Change to:
*/ }