How can i put a variable month in date function ?

```
$nbrmonth = 6;

$onemonth = date('Y-m-d H:i:s',strtotime(date("Y-m-d H:i:s", mktime()) . " + 1 month")); //work
$variabledate = date('Y-m-d H:i:s',strtotime(date("Y-m-d H:i:s", mktime()) . " + '.$nbrmonth.' month")); // not working 

```