PHP Curl error numbers (errno:26)

I recently had a hard time finding what the error numbers stand for that are returned when sending an HTTP request with php_curl.
I was looking for error number 26. Error number 26 stands for:

CURLE_READ_ERROR (26)

There was a problem reading a local file or an error returned by the read callback.

A complete list of error codes can be found at the cURL project homepage.