PHP rsa加密失败的原因解决办法

$split = str_split($data, 117);
$crypto = '';
foreach ($split as $chunk) {
    $isOkay = openssl_private_encrypt($chunk,$output,$this->_privKey);
    if(!$isOkay){
        return false;
    }
    $crypto .= base64_encode($output);
}
return $crypto;