PHP curl Unknown SSL protocol error - php

Question

We've recently moved our web application to a new server and curl requests through PHP have stopped working for HTTPS addresses (HTTP still works fine).
The old server was running openSUSE 12.3 and the new server is Ubuntu 16.04.
I've tried curl_setopt( $ch, CURLOPT_SSLVERSION, 1 ); to force TLS as suggested in answers to similar questions here, but it makes no difference.
It's all the more confusing because an HTTPS request to the same location using curl at the command line works exactly as I'd expect.
I turned on verbose output in the PHP curl call to see if that would help me and got the following output:
* Trying xxx.xxx.xxx.xxx...
* Connected to www.somesite.com (xxx.xxx.xxx.xxx) port 443 (#0)
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:#STRENGTH
* successfully set certificate verify locations:
* CAfile: /etc/ssl/certs/ca-certificates.crt
CApath: /etc/ssl/certs
* Unknown SSL protocol error in connection to www.somesite.com:443
* Closing connection 0
but when I run the command at the command line (using something like curl -d "My=post&data=here" -X POST https://www.somesite.com/... -v) I get:
* Trying xxx.xxx.xxx.xxx...
* Connected to www.somesite.com (xxx.xxx.xxx.xxx) port 443 (#0)
* found 173 certificates in /etc/ssl/certs/ca-certificates.crt
* found 696 certificates in /etc/ssl/certs
* ALPN, offering http/1.1
* SSL connection using TLS1.0 / RSA_3DES_EDE_CBC_SHA1
* server certificate verification OK
* server certificate status verification SKIPPED
* common name: *.somesite.com (matched)
* server certificate expiration date OK
* server certificate activation date OK
* certificate public key: RSA
* certificate version: #3
* subject: C=****,ST=****,L=****,O=****,OU=****,CN=*.somesite.com
* start date: Fri, 27 Jan 2017 00:00:00 GMT
* expire date: Wed, 26 Feb 2020 23:59:59 GMT
* issuer: C=US,O=thawte\, Inc.,CN=thawte SSL CA - G2
* compression: NULL
* ALPN, server did not agree to a protocol
...Response here...
Does anyone have any suggestions as to how to fix this, or what I can do to narrow down the cause of the issue?
UPDATE:
Here is the PHP code I'm using to try and find the cause of the problem:
$postvars="My=post&data=here";
$curl=curl_init(
"https://www.somesite.com/..."
);
curl_setopt($curl, CURLOPT_VERBOSE, true);
curl_setopt($curl, CURLOPT_STDERR, fopen('/tmp/sslerror', 'w'));
curl_setopt($curl, CURLOPT_POST, 1);
curl_setopt($curl, CURLOPT_POSTFIELDS, $postvars);
curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/x-www-form-urlencoded'));
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl, CURLOPT_TIMEOUT, 30);
$result=curl_exec($curl);
echo curl_error($curl);
curl_close($curl);
echo var_export($result, true);