Paypal Express Checkout preventing checkout from happening!

zc 2.1.0

I used to use this plugin but haven't in years however its been sitting 'dormant' if you like all this time.

All of a sudden today my cart was producing this error when attempting to go to checkout -

[12-Nov-2025 14:45:00 Australia/Sydney] PHP Fatal error: Uncaught Error: Undefined constant "HTTP_CATALOG_SERVER" in /home/...t/includes/functions/functions_exchange_rates.php:164
Stack trace:
#0 /home/.../public_html/zencart/includes/functions/functions_exchange_rates.php(92): doCurlCurrencyRequest()
#1 /home.../public_html/zencart/includes/modules/payment/paypal/paypal_currency_check.php(27): quote_ecb_currency()
#2 /home/.../public_html/zencart/includes/modules/payment/paypalwpp.php(282): paypalUSDCheck()
#3 /home/.../public_html/zencart/includes/modules/payment/paypalwpp.php(215): paypalwpp->update_status()
#4 /home/.../public_html/zencart/includes/classes/payment.php(120): paypalwpp->__construct()
#5 /home/.../public_html/zencart/includes/modules/pages/checkout_shipping/header_php.php(224): payment->__construct()
#6 /home/.../public_html/zencart/index.php(35): require('/home/funkybal/...')
#7 {main}
thrown in /home/.../public_html/zencart/includes/functions/functions_exchange_rates.php on line 164

I have removed the plugin and everything is happy again.

Just FYI, and curiousity as to why?

Take a look at https://github.com/zencartpro/zencart-german-grundsystem/issues/55
This shouldn't prevent from checkout.
It's just about the exchange rates. But who knows. If you say it prevents then it prevents.
It looks like this problem has been there for a long time. You could compare the code in the git with yours.

Strange because I have not made any changes to my site - it just suddenly went like this.

On clicking checkout the web just hung for a long time then eventually went to a white screen with those errors in the log.

This plugin has not previously been causing any errors. And as I said it was not in use. It was installed but not enabled.

I have the same fatal error for the 10th of novembre. i was sitting in front of the screen when a customer didn't finish his purchase.
He ended up at checkout confirmation.
He was Italian, so (€) no exchange neccessary but the log file says sth. about $check:

[10-Nov-2025 14:24:51 Europe/Berlin] PHP Fatal error: Uncaught Error: Call to undefined function zenDoCurlRequest() in /includes/functions/functions_exchange_rates.php:109
Stack trace:
#0 /includes/modules/payment/paypal/paypal_currency_check.php(29): quote_ecb_currency()
#1 /includes/modules/payment/paypal/tpl_ec_button.php(14): paypalUSDCheck()
#2 /includes/templates/responsive_classic/templates/tpl_shopping_cart_default.php(39): include('...')
#3 /includes/templates/responsive_classic/common/tpl_main_page.php(179): require('...')
#4 /index.php(95): require('...')
#5 {main}
thrown in /includes/functions/functions_exchange_rates.php on line 109

I'm going to compare the code from the git. I'm not sure whether I changed that or not.
webchills recommends to use the PayPal Checkout REST API.
If you have problems this might be a better solution for you than trying to figure out errors in case of the deprecated classic NVP/SOAP API.

I looked it up now and I wasn't aware of it prevents from checkout.
I changed it to the version from the git in May already.
But I must admit I'm at a slightly modified 157h (zen cart pro) version and not at 157i as webchills recommends.
The functions change may work but isn't compatible with PHP 8.2 (which is the recommended PHP version for 157h) ?

If I had the US version I would have swapped to the REST API.
For zen cart pro it is available for 157j only.

A new major zen cart pro release has been announced by the end of the year so I'm going to upgrade by then. This will bring the REST API integration by default.

edit: I have only one currency set at zen cart

Shop Suey:

I looked it up now and I wasn't aware of it prevents from checkout.
I changed it to the version from the git in May already.
But I must admit I'm at a slightly modified 157h (zen cart pro) version and not at 157i as webchills recommends.
The functions change may work but isn't compatible with PHP 8.2 (which is the recommended PHP version for 157h) ?

If I had the US version I would have swapped to the REST API.
For zen cart pro it is available for 157j only.

A new major zen cart pro release has been announced by the end of the year so I'm going to upgrade by then. This will bring the REST API integration by default.

edit: I have only one currency set at zen cart
Shop Suey, note that the zen cart pro version supports various countries in Europe but doesn't reflect the as-released Zen Cart versions.

I'm aware of that but one is based on the other.
And both do have the same problem under certain circumstances.

Anything here could make problems for zen cart?
PHP disable_functions:

dl, disk_free_space, diskfreespace, stream_socket_sendto, proc_get_status, proc_nice, proc_open, proc_terminate, proc_close, popen, curl_multi_exec, pcntl_exec, pcntl_fork, pcntl_setpriority, link, posix_kill, posix_mkfifo, posix_setsid, posix_setuid, posix_setpgid, posix_getpwuid, show_source, highlight_file, syslog, openlog, define_syslog_variables, apache_child_terminate, apache_setenv, apache_note