here is my query to update column leave

UPDATE page_views SET leave = NOW() WHERE id = '$id'

it gives error

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'leave = NOW()

how to solve this??

so presumably this is a PHP problem
first off you would be better supplying the relavant lines of PHP code
ie where you set the SQL upto and including the execution of the SQL.

on the face of it the SQL looks OK
I'd want to check the table name are correctly spellt and CaPiTaLiSeD. On windows it doesn't matter (on column names, but it does on table names oddly), on *NIX it matters at all times
Id want to check the column is defined as a datetime
Id want to check that the NOW function is valid for the version of MySQL I was using
Id want to check that I wasn't using a MySQL reserved word

I'm a little suspicious of your where clause.. because Im used to numeric columns with a suffix of "ID", and therefore the ' would be suprerfluous

the error occurs at the word "leave" (right where the message says)

LEAVE is a reserved word, so you have to backtick it

UPDATE page_views SET `leave` = NOW() WHERE id = $id

healdem is right about not putting quotes around values being compared with or inserted into numeric columns -- mysql will do an implicit conversion, but other database systems are not as forgiving and will toss an error there, too