Date addition using Interval

Hi Experts,

I try to write the following in my Function and it works:

ld_FromDateTemp := ld_FromDate + ''5 DAYS''::interval;

But when I have to pass a parameter in the NUMBER of days(instead of 5)
like 'li_NoOfDays'

ld_FromDateTemp := ld_FromDate + ''li_NoOfDays DAYS''::interval;

Does not work. Error as follows:
------------------------------------------------------------------------
--------
WARNING: Error occurred while executing PL/pgSQL function
proc_dailyutilization
WARNING: line 41 at assignment
ERROR: Bad interval external representation 'li_NoOfDays DAYS'
------------------------------------------------------------------------
--------
If I write this way:
ld_FromDateTemp := ld_FromDate + li_NoOfDays ''DAYS''::interval;

Following Error:
------------------------------------------------------------------------
--------
WARNING: Error occurred while executing PL/pgSQL function
proc_dailyutilization
WARNING: line 41 at assignment
ERROR: parser: parse error at or near "'DAYS'" at character 21
------------------------------------------------------------------------
--------

I am sure there is a way of doing it. Can anyone point out where I am
going wrong?

Thanks in Advance.

Regards,
Sumita