help on phrets search query

edit

I have been using two phrets parameters as follows: $rets_modtimestamp_field = "L_ListOffice1"; $rets_status_field = "L_StatusCatID";

But when I change the $rets_modtimestamp_field to any other parameter I get nothing. (originally it was the parameter to pull all properties with a listing date >= to the $rets+modtimestamp_filed. I found that if I changed the modtimestamp_field to = L_ListOffice1 field I could pull our properties.

$query = "({$rets_modtimestamp_field}=215)";


    // run RETS search

    $search = $rets->SearchQuery("Property", $class, $query, array('Limit' => 1000));

I would appreciate any insight you can provide, or if you could point me to any documentation that might help (preferably with examples).