sandbox paypal account transferring money to invalid accounts through php code

In my api,I'm using a php code to automatically transfer money amount from one sandbox paypal account to another sandbox paypal account.But through this php code , transactions are occuring for invalid sandbox user accounts which should not happen at all. furthermore , I cannot find any way to check in my code whether the reciever's sandbox account is valid or not.If it's valid ,only then tranfer money to the reciever's account.

Here is the code....

******* Paypal_class.php *******

enter code here

class Paypal {

    public function __construct($username, $password, $signature) {
        $this->username = urlencode($username);
        $this->password = urlencode($password);
        $this->signature = urlencode($signature);
        $this->version = urlencode("122.0");
        $this->api = "http://ift.tt/pI1orb";

        $string = 'cmd=' . urlencode('_notify-validate');
        //The functions can be modified but need to be urlencoded
        $this->type = urlencode("EmailAddress");
        $this->currency = urlencode("USD");
        $this->subject = urlencode("Instant Paypal Payment");
    }

    public function pay($email, $amount, $note="Instant Payment") {
        $string = "&EMAILSUBJECT=".$this->subject."&RECEIVERTYPE=".$this->type."&CURRENCYCODE=".$this->currency;

        $string .= "&L_EMAIL0=".urlencode($email)."&L_Amt0=".urlencode($amount)."&L_NOTE0=".urlencode($note);

        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $this->api);
        curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
        curl_setopt($ch, CURLOPT_VERBOSE, 1);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_FAILONERROR, true);

        curl_setopt($ch, CURLOPT_HEADER, 0);
        curl_setopt($ch, CURLOPT_POST, 1);
        $request = "METHOD=MassPay&VERSION=".$this->version."&PWD=".$this->password."&USER=".$this->username."&SIGNATURE=".$this->signature."$string";

        curl_setopt($ch, CURLOPT_POSTFIELDS, $request);
        $httpResponse = curl_exec($ch);

        if(!$httpResponse) {
            exit("MassPay failed: ".curl_error($ch).'('.curl_errno($ch).')');
        }

        $httpResponseArray = explode("&", $httpResponse);
        $httpParsedResponse = array();
        foreach ($httpResponseArray as $i => $value) {
            $tempArray = explode("=", $value);
            if(sizeof($tempArray) > 1) {
                $httpParsedResponse[$tempArray[0]] = $tempArray[1];
            }
        }

        if((0 == sizeof($httpParsedResponse)) || !array_key_exists('ACK', $httpParsedResponse)) {
            exit("Invalid HTTP Response for POST request($request) to ".$this->api);
        }

        return $httpParsedResponse;
    }

}

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