PHP schneidet ein zweidimensionales Array in einen String und entfernt doppelte Werte

// select rent_contract_id,rent_contract_parent_id from tb_rent_contract where rent_contract_id =797 or rent_contract_parent_id = 797
$arr = array(array('rent_contract_id'=>555,'rent_contract_parent_id'=>666),array('rent_contract_id'=>777,'rent_contract_parent_id'=>555),array('rent_contract_id'=>888,'rent_contract_parent_id'=>777));
$str_arr = ""; //空字符串,便于遍历后的追加
foreach ($arr as $ke => $va) {
  $str_arr .= implode(',', $va).","; 
}
$arr_str = explode(",",$str_arr); //字符串组装为数组
$arr_str = array_unique($arr_str);//去除重复的值
$str = implode(',',$arr_str);     //分割为字符串
var_dump($str);                   //string(16) "555,666,777,888,"