How do I dynamically insert NULL into a postgres table?

[georgeir]

I have a table like this \[code\]CREATE TABLE test(a int, b int);\[/code\]The values to the table are inserted from dynamic input$a, $b (PHP variables)\[code\]$query = "UPDATE test set a=$a where b = $b";pg_query($db, $query);\[/code\]This doesn't work when $a is empty (i.e when the user doesn't enter any value for a form field from the table). How to I get around this?