Php二维码不适用于长字符串

我正在尝试将我的xml字符串转换为QR码。但它没有显示任何内容。它适用于最小长度的字符串。有什么方法可以做到这一点吗？？这是我的密码

    <?php
    ob_start();
    include ('../includes/phpqrcode/qrlib.php');

    QRcode::png('<?xml version="1.0" encoding="UTF-8" ?>
<vlsm>
<facility><facility_id>020175</facility_id>
<facility_name>Freda Rebecca Mine Clinic</facility_name>
<facility_state>Mashonaland Central</facility_state>
<facility_district>Bindura</facility_district>
<facility_hub_name></facility_hub_name>
<facility_sample_id></facility_sample_id>
<facility_request_clinician_name></facility_request_clinician_name>
<facility_request_clinician_phone_number></facility_request_clinician_phone_number>
<facility_support_partner></facility_support_partner>
<facility_physician_name></facility_physician_name>
<facility_date_test_ordered_by_physician>0000-00-00</facility_date_test_ordered_by_physician>
<facility_sample_collection_date>2017-03-06 06:00:00</facility_sample_collection_date>
<facility_sample_collected_by></facility_sample_collected_by>
<facility_mobile_numbers></facility_mobile_numbers>
<facility_emails></facility_emails>
<facility_test_urgency></facility_test_urgency>
</facility></vlsm>', "test.png", "L", 4, 4);

---

当您直接将其输出到浏览器时会发生什么？它有时会显示二维码，但有时不会显示任何内容。甚至不会抛出错误。它在扫描生成的二维码时只显示最小内容。这就是问题所在@RST