PHP实现二维数组转字符串

$arr = array(  
[0] => array( [rent_contract_parent_id] => 0 ),
[1] => array( [rent_contract_parent_id] => 645 ),
[2] => array( [rent_contract_parent_id] => 665 ) 
);

//二维数组转化为字符串，中间用,隔开    
    function toString($arr){    
        foreach ($arr as $value){    
            
            $value = join(",",$value);   
            
            $temp[] = $value;    
        }    
        foreach($temp as $v){    
            
            $str.=$v.",";    
        }    
        $str = substr($t,0,-1);  //利用字符串截取函数消除最后一个逗号    
        return $str;    
    }