[PHP] converting seconds since unix epoc to date array

[Jason FB]

Can anyone tell me how to convert a date stored in the format 
"YYYY-MM-DD" to an integer of seconds since unix epoc,

then add $daysToAdd days to this value (I suppose if the integer was 
a number of seconds it would have to be $daysToAdd*60*60*24 to get 
the number of seconds to add)

then convert the new value to a timestamp which I can output using the function
string date ( string format [, int timestamp])
I tried doing this with mktime() and couple different ways using 
strtotime() and also with strftime() but I kept getting -1 as the 
result for strtotime() indicating the function couldn't interpret the 
given date.

I've been through all the date-related functions in the manual I can 
think of, but I'm stumpped on this one...