Custom PHP to verify login

[webix]

Hello folks!

I am developping a custom control panel for my users to manage their IP addresses (rDNS, etc...).
I want they use their email address and password from WHMCS to login on the panel.

In order to do that, i made a login form that will pass the POST data (email and password hashed in MD5) to the following function:

function validate_login($email, $password) {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, 'https://XXX.XXXXXXX.XXX/includes/api.php');
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS,
        http_build_query(
            array(
                'action' => 'ValidateLogin',
                // See https://developers.whmcs.com/api/authentication
                'username' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
                'password' => 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXxxXX',
                'email' => $username,
                'password2' => $password,
                'responsetype' => 'json',
            )
        )
    );
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    return curl_exec($ch);
    curl_close($ch);
}

The problem is that the function returns in error with the message "Email or Password Invalid".

Anyone can help me out to find the error?

Best regards