Keep getting 503 on Curl request

503 means the service is temporarily not available. There should be a Retry-After header in the response telling you, when you can retry the operation.

Just retry it. Otherwise ask the administrator of that machine, why the service is not available.

I don't know whether cURL throws Exception.  I've always tested the response with something like this code snippet.  You might give this a try and show us what you get back...

<?php // /demo/curl_get_response_object_example.php
/**
 * Demonstrate the basics of cURL GET-method request
 *
 * http://curl.haxx.se/libcurl/c/libcurl-errors.html
 */
error_reporting(E_ALL);


// USAGE EXAMPLE: BECAUSE IT IS ON MY SERVER, I HAVE HARD-CODED THIS
$url = 'https://twitter.com/RayPaseur';

// TRY THE REMOTE WEB SERVICE
$response = new GET_Response_Object($url);

// SHOW THE WORK PRODUCT
echo "<pre>";
if (!$response->document) var_dump($response); // ON FAILURE
echo htmlentities($response->document); // ON SUCCESS

// SHOW THE COOKIES, IF ANY
echo PHP_EOL;
echo file_get_contents('cookie.txt');


Class GET_Response_Object
{
    public $href, $title, $http_code, $errno, $info, $document;

    public function __construct($href, $user=NULL, $pass=NULL, $get_array=[], $title=NULL)
    {
        // ACTIVATE THIS TO AVOID TIMEOUT FOR LONG RUNNING SCRIPT
        // set_time_limit(10);

        // STORE THE CALL INFORMATION
        $this->href  = $href;
        $this->title = $title;

        // PREPARE THE GET STRING
        $get_string = http_build_query($get_array);
        if ($get_string) $get_string = '?' . $get_string;

        // MAKE THE REQUEST
        if (!$response = $this->my_curl($href, $user, $pass, $get_string))
        {
            // ACTIVATE THIS TO SEE THE ERRORS AS THEY OCCUR
            trigger_error("Errno: $this->errno; HTTP: $this->http_code; URL: $this->href", E_USER_WARNING);
        }
        else
        {
            return $response;
        }
    }

    protected function my_curl($url, $user, $pass, $get_string, $timeout=3)
    {
        // PREPARE THE CURL CALL
        $curl = curl_init();

        // HEADERS AND OPTIONS APPEAR TO BE A FIREFOX BROWSER REFERRED BY GOOGLE
        $header[] = "Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5";
        $header[] = "Cache-Control: max-age=0";
        $header[] = "Connection: keep-alive";
        $header[] = "Keep-Alive: 300";
        $header[] = "Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7";
        $header[] = "Accept-Language: en-us,en;q=0.5";
        $header[] = "Pragma: "; // BROWSERS USUALLY LEAVE THIS BLANK

        // SET THE CURL OPTIONS - SEE http://php.net/manual/en/function.curl-setopt.php
        curl_setopt( $curl, CURLOPT_URL,            $url . $get_string  );
        curl_setopt( $curl, CURLOPT_USERAGENT,      'Mozilla/5.0 (Windows NT 6.1; rv:49.0) Gecko/20100101 Firefox/49.0'  );
        curl_setopt( $curl, CURLOPT_HTTPHEADER,     $header  );
        curl_setopt( $curl, CURLOPT_REFERER,        'https://www.google.com'  );
        curl_setopt( $curl, CURLOPT_ENCODING,       'gzip,deflate'  );
        curl_setopt( $curl, CURLOPT_AUTOREFERER,    TRUE  );
        curl_setopt( $curl, CURLOPT_RETURNTRANSFER, TRUE  );
        curl_setopt( $curl, CURLOPT_FOLLOWLOCATION, TRUE  );
        curl_setopt( $curl, CURLOPT_TIMEOUT,        $timeout  );
        curl_setopt( $curl, CURLOPT_HTTPAUTH,       CURLAUTH_ANY );
        curl_setopt( $curl, CURLOPT_USERPWD,        "$user:$pass" );

        // GET GOOD ERROR MESSAGES
        curl_setopt( $curl, CURLOPT_VERBOSE,        TRUE   );
        curl_setopt( $curl, CURLOPT_FAILONERROR,    TRUE   );

        // IF USING SSL, THIS INFORMATION MAY BE IMPORTANT
        // http://php.net/manual/en/function.curl-setopt.php#110457
        // http://php.net/manual/en/function.curl-setopt.php#115993
        // http://php.net/manual/en/function.curl-setopt.php#113754
        // REDACTED IN 2015 curl_setopt( $curl, CURLOPT_SSLVERSION, 3 );
        curl_setopt( $curl, CURLOPT_SSL_VERIFYHOST, FALSE  );
        curl_setopt( $curl, CURLOPT_SSL_VERIFYPEER, FALSE  );

        // SET THE LOCATION OF THE COOKIE JAR (THIS FILE WILL BE OVERWRITTEN)
        curl_setopt( $curl, CURLOPT_COOKIEFILE,     'cookie.txt' );
        curl_setopt( $curl, CURLOPT_COOKIEJAR,      'cookie.txt' );

        // RUN THE CURL REQUEST AND GET THE RESULTS
        $this->document  = curl_exec($curl);
        $this->errno     = curl_errno($curl);
        $this->info      = curl_getinfo($curl);
        $this->http_code = $this->info['http_code'];
        curl_close($curl);

        return $this;
    }
}

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Interestingly enough I have the same issue with using file_put_contents:-

set_time_limit(0);
file_put_contents("test-dwn.xls",file_get_contents("https://myUsername:myPassword@myInstance.service-now.com/myTable.do?EXCEL"));

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Works fine still on my machine though :-S

Agree with @gr8gonzo about CURLOPT_SSL_VERIFYHOST and CURLOPT_SSL_VERIFYPEER.  Setting these to FALSE is an expedient for one-time runs, not something to use in a deployed application.  

If your PHP installation doesn't have an up-to-date CA root certificate bundle, consider downloading the one at the curl website.

Check your cURL version; you want to be using using cURL 7.34 or newer.

Were you able to test the "get_response_object" code sample posted above?  We have to rely on you to test and get the results back to us; we do not have your credentials to make the tests.

Weirdly enough this just started to work, after multiple moans to my hosting provider :-S