API issue - empty fields, PHP

We have the problem while querying rows with condition “some field is NULL”. The part of the code that does not work and causes the problem below:

$conditionIssue = new stdClass();

$fld = new stdClass();

$fld->FieldName = "Parent";

$conditionIssue->LeftExpression = new SoapVar($fld, SOAP_ENC_OBJECT, 'FieldExpression', $this->soapQApiUrl);

$conditionIssue->Operator = "Equal";

$conditionIssue->RightExpression = null;

$parent_null = new SoapVar($conditionIssue, SOAP_ENC_OBJECT, 'Compare', $this->soapQApiUrl);

I wonder if you can advise how we can select entities without parent in php – as an example we need to get list of projects without parents.