Here you go:

function formatDocument($doc, $csl=Null, $textonly=False, $showcover=False, $showlink=False) {
			$result = '';

                        // format document with a given CSL style and the CiteProc.php
                        if ($csl != Null && class_exists("citeproc")){
                        	// read the given CSL style from XML document, load it to a string, and convert it to an object
				$cacheid = "csl-".$csl;
				$csl_file = $this->getOutputFromCache($cacheid);
				if (empty($csl_file)) {
				        $curl = curl_init($csl);
					curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
					
					$http_headers = array(
						'User-Agent: Junk', // Any User-Agent will do here
					);
					curl_setopt($curl, CURLOPT_HEADER, true);
					curl_setopt($curl, CURLOPT_HTTPHEADER, $http_headers);
					
                        		$csl_file = curl_exec($curl);
					if (curl_getinfo($curl,CURLINFO_HTTP_CODE) < 400) {
					   if ($csl_file !== false) {
					      $pos_body = stripos($csl_file, "<?xml version");
					      $csl_file = substr($csl_file, $pos_body);
					      $this->updateOutputInCache($cacheid, $csl_file);
					   } else {
					      echo "<p>1 Mendeley Plugin Error: Failed accessing Menedley API: " . curl_error($curl) . "</p>";
					   }
					} else {
					   echo "<p>2 Mendeley Plugin Error: Failed accessing Mendeley API: " . curl_getinfo($curl,CURLINFO_HTTP_CODE) . "||||" . $csl_file . "</p>";
					   $csl_file = "";
					}
					curl_close($curl);
				}
                        	$csl_object = simplexml_load_string($csl_file);

It works! Thanks @kochm!

BTW, please credit @dinel2016 for the code. I just passed it along.