There are multiple methods for getting current URL parameters. The following seem to work fine.
filter_input — Gets a specific external variable by name and optionally filters it . Works on PHP 5 >= 5.2.0.
The following code gets the districts from the url provided in the parameter field_district_ref_nid. The filter input converts the result into an array.
<?php
$districts = filter_input(INPUT_GET, 'field_district_ref_nid', FILTER_DEFAULT, FILTER_FORCE_ARRAY | FILTER_FORCE_ARRAY);
dpm($districts);
?>
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Source
http://php.net/manual/en/function.filter-input.php