Warning:unlink() Permission denied

function indent_groups() {
    $xml_file = "/etc/ossim/server/groups.xml";
    $tab = file($xml_file);
    unlink($xml_file);
    $fic = fopen($xml_file, 'w');
    if ($fic) {
        $nb = count($tab);
        for ($i = 0; $i <= $nb; $i++) {
            $string = str_replace("><", ">\n<", $tab[$i]);
            fwrite($fic, $string);
        }
    }
    fclose($fic);
    $tab = file($xml_file);
    unlink($xml_file);
    $fic = fopen($xml_file, 'w');
    $space = "";
    for ($i = 0; $i < count($tab); $i++) {
        if (strncmp("<group ", trim($tab[$i]) , 7) == 0 || strcmp("</group>", trim($tab[$i])) == 0) $space = "   ";
        elseif (strncmp("<append-directive ", trim($tab[$i]) , 18) == 0) $space = "      ";
        else $space = "";
        if (trim($tab[$i]) != '') {
            $string = $space . trim($tab[$i]) . "\n";
            if (strcmp("</group>", trim($string)) == 0 || strncmp("<?xml ", trim($string) , 6) == 0) $string = $string . "\n";
            fwrite($fic, $string);
        }
    }
    fclose($fic);
}