Searches using Formula(Text) in PHP Toolkit

Hi,

I've been searching high and low for examples of how to do searches using the Formula (Text) but have not been able to understand what needs to be done and would appreciate an example of how I can do something similar as the example below in PHP:

Resource Allocation Search:

Formula (Text)  contains "Project User" {resource}

Is this possible?

My starting point is:

[code]
    $search = new ResourceAllocationSearchRowBasic();

    $allocationSearchField = new SearchStringField();
    $allocationSearchField->operator = "doesNotContain";
    $allocationSearchField->searchValue = "Project User";
    $search->resource = $allocationSearchField;
[/code]

Many thanks.
Aktar