Having trouble with authentication to access API - php

Question

I am trying to retrieve information from an API. The API requires authentication in order to access it. I have the tried the following, but it says that the Authentication has failed. I already confirmed that the credentials are correct, so that is not the problem. Any suggestions?
$pro = '0000000000';
$userName = 'userName';
$password = 'password';
$userPass = $userName.':'.$password;
$host = 'https://wardtlctools.com/wardtrucking/webservices/imaging';
$post = array(
'User' => array(
'UserName' => $userName,
'Password' => $password
),
'Reference' => array(
'RefNo' => $pro,
'RefType' => 'WARDPRO',
'DelMethod' => 'Link',
'Documents' => array(
'DocType' => 'BL',
'DocType' => 'DR'
)
)
);
$ch = curl_init($host);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_TIMEOUT, 30);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
curl_setopt($ch, CURLOPT_USERPWD, $userPass);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$return = curl_exec($ch);
curl_close($ch);
print_r($return);
This is the documentation from the provider's website.