value should be a string that is the name of a valid callback function for the following values of the option parameter:
Option Set value to Notes
CURLOPT_HEADERFUNCTION The name of a callback function where the callback function takes two parameters. The first is the cURL resource, the second is a string with the header data to be written. The header data must be written when using this callback function. Return the number of bytes written.
CURLOPT_PASSWDFUNCTION The name of a callback function where the callback function takes three parameters. The first is the cURL resource, the second is a string containing a password prompt, and the third is the maximum password length. Return the string containing the password.
CURLOPT_PROGRESSFUNCTION The name of a callback function where the callback function takes three parameters. The first is the cURL resource, the second is a file-descriptor resource, and the third is length. Return the string containing the data.
CURLOPT_READFUNCTION The name of a callback function where the callback function takes two parameters. The first is the cURL resource, and the second is a string with the data to be read. The data must be read by using this callback function. Return the number of bytes read. Return 0 to signal EOF.
CURLOPT_WRITEFUNCTION The name of a callback function where the callback function takes two parameters. The first is the cURL resource, and the second is a string with the data to be written. The data must be written by using this callback function. Must return the exact number of bytes written or this will fail.