PHP Form Sending Issues (using SendInBlue) - php

Question

I have a basic PHP contact form which I use on a couple of sites, which sends email using the SendInBlue API.
My problem is that I have the form working perfectly on 1 site, now I am using the EXACT same code for a second site and just changing the email, name, etc - however when testing it I now get this error:
Fatal error: Call to undefined method Mailin::send_email() in
/URL/contactpage.php on line 71
FYI, line 71 is:
$mailin->send_email($data);
I have attached the complete code below - this works perfectly on 1 site, but I get this error on my second site.
Any ideas?
Thank you!
<?php
//Email Details for Form Notifications
$email_to = 'Test#test.com'; //the address to which the email will be sent
$email_to_name = 'Test';
//Form Fields
$fname = $_POST['firstname'];
$lname = $_POST['lastname'];
$email = $_POST['email'];
$fullmessage = $_POST['message'];
//Subject Lines
$subject_to = 'Test';
$subject_touser = 'Test';
//URL Redirects
$confirm = 'TestConfirm';
$errorurl = 'TestError';
//Validate
$emailval = filter_var( $email, FILTER_VALIDATE_EMAIL );
if ($emailval == false)
{
header("Location: $errorurl");
} else {
// The email to us
$message_to = 'Message here';
//
// The email to them
$message_touser = 'Message here';
//
require('Mailin.php');
//Notification Email
$mailin = new Mailin('https://api.sendinblue.com/v2.0','MY_KEY');
$data = array( "to" => array($email_to=>$email_to_name),
"cc" => array($email_to_cc=>$email_to_cc_name),
"from" => array($email,$fname),
"subject" => $subject_to,
"html" => $message_to
);
$mailin->send_email($data);
//Email to User
$mailin = new Mailin('https://api.sendinblue.com/v2.0','MY_KEY');
$data2 = array( "to" => array($email=>$fname),
"from" => array($email_to,$email_to_name),
"subject" => $subject_touser,
"html" => $message_touser
);
$mailin->send_email($data2);
header("Location: $confirm");
}
?>

Answer 1

I was using a deprecated Mailin.php. Updated the file and everything works now.