function sendWebhook($data) {
    $url = 'https://url/';
    $jsonDataEncoded = json_encode($data);
    $webhook = curl_init($url);
    curl_setopt($webhook, CURLOPT_POST, 1);
    curl_setopt($webhook, CURLOPT_POSTFIELDS, $jsonDataEncoded);
    curl_setopt($webhook, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
    $result = curl_exec($webhook);
}

$data = array(
    'key' => 'value'
);

sendWebhook($data);

Source: http://thisinterestsme.com/sending-json-via-post-php/