How to convert cURL to PHP cURL

Couple of nits to pick in your code aside from the obvious typo of CURLOPT_POST. You use filter_var but carry on if the filter fails; you should check for a false value. You should not be using urldecode on the URL, the whole idea of http_build_query is that it does the URL encoding for you. This one's a personal preference but I find curl_setopt_array much easier to work with. And, you should be checking if your result is valid JSON and use proper headers when sending the response back to jQuery.

header("Content-Type: application/json");$email  = filter_var($_POST["Email"], FILTER_SANITIZE_EMAIL);if (!$email) {    header("HTTP/1.1 500 Server Error");    $res = ["result"=>false, "message"=>"Invalid email"];    echo json_encode($res);    exit;}$fields = ['domain' => $email];$url = "https://mailcheck.p.mashape.com/?" . http_build_query($fields);$ch = curl_init($url);curl_setopt_array($ch, [    CURLOPT_RETURNTRANSFER => true,    CURLOPT_HTTPHEADER => [        'X-Mashape-Key: f123456879',        'Accept: application/json'    ]]);$server_output = curl_exec($ch);curl_close ($ch);if (json_decode($server_output)) {    echo $server_output;} else {    header("HTTP/1.1 500 Server Error");    $res = ["result"=>false, "message"=>"Invalid response from server: $server_output"];    echo json_encode($res);}