Error code 1582: Incorrect parameter count in the call to native function 'FROM_UNIXTIME'

Error code 1582: Incorrect parameter count in the call to native function
'FROM_UNIXTIME'

My code is like:
$current = time();
$nexttime = mktime($hour,$minute,$second,$month,$day,$year);
//$hour...from a table
and mySQL statement is like:
INSERT INTO table(...) values
('FROM_UNIXTIME(".$current.")',FROM_UNIXTIME(".$nexttime."));
I found the first FROM_UNIXTIME(".$current.") is accepted by mySQL, while
the second FROM_UNIXTIME(".$nexttime.") shows:
Error code 1582: Incorrect parameter count in the call to native function
'FROM_UNIXTIME'.
Any suggestion is appreciated.