How to UPDATE a certain row OR RETURN FALSE if it doesn't exist?

Question

How to do it in one query?

Answer 1

You can do something like:

function updateValue()
{
     mysql_query($sql); // your update goes here
     return mysql_affected_rows() > 0;
}

From BoltClock's comment:

Bear in mind that mysql_affected_rows() returns 0 if a row exists but the old and new values are equal (meaning there was no need for an update).

Answer 2

You may use mysql_affected_rows() function to check, whether any rows were affected by update. See for details: http://php.net/manual/en/function.mysql-affected-rows.php