php running out of memory while constructing large array from database - php

Question

I have a project were i need to pull up to 50,000 records from a database and create a csv file with that data. My problem is that the server will only return 300 records at a time. How can i effectively run this script apx 166 times while storing each response from the server in an array without running out of memory?
Once the last call request the server has been made, i will run the function (not shown) to create the CSV from the array.
class RetrieveData {
public $fields = array("Name1", "Name2", "Name3", "Name4");// List of names to pull from database
public $batchSize; //max 300 records returned at a time
public $NextPage;
public $nextPageToken; //token returned from previous call for paging
public function getData(){
$url = $this->host . "/rest/v1/list/" . $this->listId . "/leads.json?access_token=" . $this->getToken();
if (isset($this->fields)){
$url = $url . "&fields=" . $this::csvString($this->fields);
}
if (isset($this->batchSize)){
$url = $url . "&batchSize=" . $this->batchSize;
}
if (isset($this->nextPageToken)){
$url = $url . "&nextPageToken=" . $this->nextPageToken;
}
$ch = curl_init($url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('accept: application/json',));
$response = curl_exec($ch);
return $response;
}
private function getToken(){
$ch = curl_init($this->host . "/identity/oauth/token?grant_type=client_credentials&client_id=" . $this->clientId . "&client_secret=" . $this->clientSecret);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('accept: application/json',));
$response = json_decode(curl_exec($ch));
curl_close($ch);
$token = $response->access_token;
return $token;
}
private static function csvString($fields){
$csvString = "";
$rr= implode(",",$fields);
$FF = str_replace(' ', '', $rr);
$csvString = $FF;
$csvString = rtrim($csvString,',');
}
return $csvString;
}
}