处理php mktime和date时会出现奇怪的结果

我需要根据商店的开业时间加上偏移量生成交货时间。打开时间生成正确，偏移量生成正确，但我似乎无法正确组合它们。我已经尝试了几种方法：

$open = array('open_hr' => 14, 'open_min' => 0);
$tmpTime = mktime($open['open_hr'], $open['open_min']);
$openTime = date('G:i', $tmpTime);
$tmpOffset = 90; //in minutes
$tmpOpenWithOffset = $tmpTime + $tmpOffset * 60;
$finalTime = date('G:1', $tmpOpenWithOffset);
echo "open time: " . $openTime . "; offset in minutes: " .$tmpOffset . "; final time: " .$finalTime;

这样做的结果是：

开放时间：14:00；偏移量（分钟）：90；最终时间：15:1

我预计14:00加90分钟会得到15:30。

我也尝试过这种方式：

$open = array('open_hr' => 14, 'open_min' => 0);
$tmpTime = mktime($open['open_hr'], $open['open_min']);
$openTime = date('G:i', $tmpTime);
$tmpOffset = 90;
$finalTime = strtotime('+'.$tmpOffset . ' minutes', $openTime);
echo "open time: " . $openTime . "; offset in minutes: " .$tmpOffset . "; final time: " .$finalTime;

这导致：

open time: 14:00; offset in minutes: 90; final time: 5414

我还尝试使用eval来评估strtotime的第一个参数，如下所示：

$open = array('open_hr' => 14, 'open_min' => 0);
$tmpTime = mktime($open['open_hr'], $open['open_min']);
$openTime = date('G:i', $tmpTime);
$tmpOffset = 90;
$tmpStr = eval('+'.$tmpOffset . ' minutes');
$finalTime = strtotime($tmpStr, $openTime);
echo "open time: " . $openTime . "; offset in minutes: " .$tmpOffset . "; final time: " .$finalTime;

这导致：

Parse error: syntax error, unexpected $end in /t.php(7) : eval()'d code on line 1
open time: 14:00; offset in minutes: 90; final time: 

同样，所需的结果是，将90分钟添加到14:00将返回15:30。我在这里做错了什么？