Sending empty values with PDO results in error

We have something like the following PDO Statement which we use to communicate with a PostgreSQL 8.4 DB.

$st = $db -> prepare("INSERT INTO Saba.Betriebskosten (personalkosten)
                                            VALUES(:kd_personalkosten)");
$st -> bindParam(':kd_personalkosten', $val['kd_personalkosten']);

$val['kd_personalkosten'] is either empty/null or contains a double value. In the case it is empty/null, we just want to insert an empty value, but we receive the following error:

SQLSTATE[22P02]: Invalid text representation: 7 ERROR:  invalid input syntax for type double precision: '';

Which means that empty/null is converted to an empty STRING which is not compatible with the double precision field. How to circumvent this error?

it seems to me that value is "" (empty string) which bindParam converts to "" in SQL query, and since personalkosten is of type Double it raises the error.

This should fix this issue with empty text to double conversion:

$st -> bindParam(':kd_personalkosten', (float) $val['kd_personalkosten']);

If you would really want to insert NULL value when variable is empty then you should do this:

$value = $val['kd_personalkosten'];
if ($value === '' or $value === NULL) {
    $st->bindValue(':kd_personalkosten', NULL, PDO::PARAM_NULL); // note the bindValue() instead of bindParam()
} else {
    $st->bindParam(':kd_personalkosten', $value);
}

About bindValue vs bindParam from php manual:

bindParam()

Binds a PHP variable to a corresponding named or question mark placeholder in the SQL statement that was used to prepare the statement. Unlike PDOStatement::bindValue(), the variable is bound as a reference and will only be evaluated at the time that PDOStatement::execute() is called.

Most parameters are input parameters, that is, parameters that are used in a read-only fashion to build up the query. Some drivers support the invocation of stored procedures that return data as output parameters, and some also as input/output parameters that both send in data and are updated to receive it.

Basically bindValue allows you to bind a direct value or constant, whilst bindParam requires a variable or reference to be passed in.