That is the right version, I think set_include_path may be disabled on your server. Create a new php file with this test script and post the output.
<?php
echo ini_get('include_path') .'
';
$include_path= 'test/path:';
$include_path.= ini_get('include_path');
if (set_include_path($include_path) === false) {
die("Fatal error: ini_set/set_include_path does not work.");
} else {
echo ini_get('include_path') .'
';
}
?>