$post = [
        'message' => $wiadomosc,
        'from' => 'ECO',
        'bulkVariant'   => "ECO",
        'to'   => $numer_telefonu,
    ];

    $ch = curl_init('https://justsend.pl/api/rest/message/send/'.JUSTSEND_API);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
    $response = curl_exec($ch);
    var_dump($response);
    curl_close($ch);