Turns out there was a bug in the 2019_1 PHP toolkit. In the SOAP client constructor, it was doing this:

$wsdl = NS_HOST . "wsdl/v" . NS_ENDPOINT . "_0/netsuite.wsdl";
...
$options['location'] = NS_HOST . "/services/NetSuitePort_" . NS_ENDPOINT;

We had a trailing slash in the NS_HOST constant, as without it the wsdl URL was wrong, and it wouldn't load the wsdl. But this meant that the location header ended up with a double slash in the URL. Looks like 2019_1, this worked ok, but in 2019_2, it doesn't. Netsuite have fixed this in their 2019_2 version of the PHP toolkit.