updating integer column

hi all, i have a table with an int8 column.
my PHP script should update a row with the new values in the form.
it works fine if i put a number in the field but gives this error if left
blank:

Warning: pg_exec() query failed: ERROR: Bad int8 external representation "" in
/home/httpd/htdocs/forms/manage/update_profile.php on line 140

how can i get around this..
oh and what is the best datatype to use for a 10 digit phone number?.

any help appreciated.
thanks
   - bruce

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