This is a sample of PHP script uploading files to the Panel. Change HOST, LOGIN, PASSWD, and FILENAME with Plesk credentials.

HOST The IP address or name of the Panel-managed server.
LOGIN Login name of the Panel administrator.
PASSWD Password of the Panel administrator.
FILENAME Full name of the file to be uploaded. 
<?php
define("HOST", "10.58.97.31");
define("PATH", "/enterprise/control/agent.php");
define("LOGIN", "admin");
define("PASSWD", 'setup');
define("FILENAME", '../../data.rpm');

function write_callback($ch, $data)
{
echo $data;
return strlen($data);
}

function uploadFile($filename)

{$url = "https://" . HOST . ":8443" . PATH;

$headers = array(
"HTTP_AUTH_LOGIN: " . LOGIN,
"HTTP_AUTH_PASSWD: " . PASSWD,
"HTTP_PRETTY_PRINT: TRUE",
"Content-Type: multipart/form-data;",
);

// Initialize the curl engine
$ch = curl_init();

// Set the curl options
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
// this line makes it work under https
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);

// Set the URL to be processed
curl_setopt($ch, CURLOPT_URL, $url);

curl_setopt($ch, CURLOPT_POSTFIELDS, array
('sampfile'=>"@$filename"));

$result = curl_exec($ch);

if (curl_errno($ch)) {
echo "\n\n-------------------------\n" .
"cURL error number:" .
curl_errno($ch);
echo "\n\ncURL error:" . curl_error($ch);
}

curl_close($ch);

//fclose($fp);

return;
}

uploadFile(realpath(FILENAME));

?>