Be careful when using curl_exec() and the CURLOPT_RETURNTRANSFER option. According to the manual and assorted documentation:
Set CURLOPT_RETURNTRANSFER to TRUE to return the transfer as a string of the return value of curl_exec() instead of outputting it out directly.
When retrieving a document with no content (ie. 0 byte file), curl_exec() will return bool(true), not an empty string. I've not seen any mention of this in the manual.
Example code to reproduce this:
<?php
// fictional URL to an existing file with no data in it (ie. 0 byte file)
$url = 'http://www.example.com/empty_file.txt';
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HEADER, false);
// execute and return string (this should be an empty string '')
$str = curl_exec($curl);
curl_close($curl);
// the value of $str is actually bool(true), not empty string ''
var_dump($str);
?>