Stripe API - PHP Curl request behind a proxy - php

Question

I'm trying to make a request behind a proxy to stripe api using php and curl. The following works if I'm not behind a proxy:
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://api.stripe.com/v1/balance");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_USERPWD, "$a_pass");
$out = curl_exec($ch); # returns my balance
curl_close($ch);
A request to any other domain using the proxy works:
$username = 'user';
$password = 'pass';
$proxy = 'proxy:port';
$ch = curl_init('https://api64.ipify.org?format=json');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_PROXY, $proxy);
curl_setopt($ch, CURLOPT_PROXYUSERPWD, "$username:$password");
$result = curl_exec($ch); # returns the proxy ip
curl_close($ch);
But if I mix both, It doesn't work and a 403 error is returned by stripe api:
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://api.stripe.com/v1/balance");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_USERPWD, "$a_pass");
curl_setopt($ch, CURLOPT_FAILONERROR, true);
curl_setopt($ch, CURLOPT_PROXY, $proxy);
curl_setopt($ch, CURLOPT_PROXYUSERPWD, "$username:$password");
$out = curl_exec($ch);
if (curl_errno($ch)) {
print_r($curl_error($ch)); # 403
}
curl_close($ch);
It seems the CURLOPT_USERPWD is lost if the request is made using a proxy.
Any idea how to use a CURLOPT_PROXYUSERPWD, CURLOPT_PROXY and CURLOPT_USERPWD sucessfully on the same request?
Notes:
I cannot install any additional software on this machine.