Loop a variable inside an sql query - php

Question

I'm trying to work out the following,
I am querying a Salesforce db with PHP, an seems that they don't really allow relationship queries, which is my whole problem, so need to run 2 queries, a query first to populate a variable that is used into a second query right after.
That variable will hold a string with usernames, just plain text usernames, that I need to match with an Id that will match a OwnerId after,
something like this,
<?php
$query = "SELECT FirstName FROM User WHERE Id in (SELECT OwnerId from Case WHERE Region__c = 'WORLD' and CaseOwnerManager__c = 'some_guy' AND Status = 'Open')";
$response = $mySforceConnection->query($query);
$queryResult = new QueryResult($response);
for ($queryResult->rewind(); $queryResult->pointer < $queryResult->size; $queryResult->next()) {
$record = $queryResult->current();
$firstName = $record->fields->FirstName;
echo $firstName."<br>\n";
}
$query = "SELECT CaseNumber, Status FROM Case where Status = 'Open' and OwnerId in (SELECT Id FROM User where FirstName = '{$firstName}')";
$response = $mySforceConnection->query($query);
$queryResult = new QueryResult($response);
for($queryResult->rewind(); $queryResult->pointer < $queryResult->size; $queryResult->next()) {
$record = $queryResult->current();
$caseNumber = $record->fields->CaseNumber;
$status = $record->fields->Status;
echo $caseNumber."<br>\n";
echo $status."<br>\n";
}?>
Of course, in the above, the second query, the one with the $firstName variable, it picks up the last value of the string and then the loop gives the CaseNumber and Status of that single user.
So what i need to do is get that second query in some sort of loop and while it is looping, get the $firstName variable to loop as well.
So at the end, $firstName should populate the th of a table and $caseNumber and $status its tds
Any ideas on how to do it, or if it even can be done?
Cheers.