Executing Curl with PHP - php

Question

I'm trying to use Docverter to convert LaTeX/markdown files to PDF but am having trouble using PHP to do CURL to access Docverter via their API. I'm know I'm not a total idiot b/c i can get this to work adapting the shell script in this Docverter example and running from command line (Mac OSX).
Using PHP's exec():
$url=$_SERVER["DOCUMENT_ROOT"];
$file='/markdown.md';
$output= $url.'/markdown_to_pdf.pdf';
$command="curl --form from=markdown \
--form to=pdf \
--form input_files[]=#".$url.$file." \
http://c.docverter.com/convert > ".$output;
exec("$command");
This gives no error messages but doesn't work. Is there a path issue somewhere?
UPDATE Based on #John's suggestion, here's an example using PHP's curl_exec() adapted from here. Unfortunately this also doesn't work though at least it gives error messages.
$url = 'http://c.docverter.com/convert';
$fields_string ='';
$fields = array('from' => 'markdown',
'to' => 'pdf',
'input_files[]' => $_SERVER['DOCUMENT_ROOT'].'/markdown.md',
);
//url-ify the data for the POST
foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
rtrim($fields_string, '&');
//open connection
$ch = curl_init();
//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_URL, $url);
curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);
//execute post
$result = curl_exec($ch);
//close connection
curl_close($ch);

Answer 1

I solved my own problem. There were two main problems with the above code:
1) The $fields array was incorrectly formatted for the input_files[]. It needed a #/ and mime-type declaration (see code below)
2) The curl_exec() output (the actual newly created file contents) needed to be returned and not just true/false which is this function's default behavior. This is accomplished by setting the curl option curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); (see code below).
Full working example
//set POST variables
$url = 'http://c.docverter.com/convert';
$fields = array('from' => 'markdown',
'to' => 'pdf',
'input_files[]' => "#/".realpath('markdown.md').";type=text/x-markdown; charset=UTF-8"
);
//open connection
$ch = curl_init();
//set options
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-type: multipart/form-data"));
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); //needed so that the $result=curl_exec() output is the file and isn't just true/false
//execute post
$result = curl_exec($ch);
//close connection
curl_close($ch);
//write to file
$fp = fopen('uploads/result.pdf', 'w'); //make sure the directory markdown.md is in and the result.pdf will go to has proper permissions
fwrite($fp, $result);
fclose($fp);