$arr =  array('messagetext' => 'This is a test', 'senderid' => 'SendMode', 'recipients' => array('0870000000'));

$url = 'https://rest.sendmode.com/v2/send';
$data = array('message' => json_encode($arr));

// use key 'http' even if you send the request to https://...
$options = array(
    'http' => array(
        'header'  => "Content-type: application/x-www-form-urlencoded\r\n" .
            "Authorization: YOUR_ACCESS_KEY\r\n",
        'method'  => 'POST',
        'content' => http_build_query($data)
    )
);
$context  = stream_context_create($options);
$result = file_get_contents($url, false, $context);