What Steps do you Take to Troubleshoot Problems with PHP cURL?

Question

Almost any working PHP programmer has faced having to use CURL to send raw HTTP requests, whether it's for credit card payment processing, nefarious screen scraping, or something in-between.

Almost any forum where PHP programmers congregate has a large number of people who can't get the cURL functions to do what they want.

When cURL isn't working for you, what troubleshooting techniques do you use to figure out why it isn't working? What weird gotchas with PHP's curl implementation have you run into? If someone asks a "HALP MY CURL IZ BROKEN" question on a forum, what are the steps you take to figure out why their request isn't working?

Answer 1

I find the CURLINFO_HEADER_OUT option to be very useful.

<?php
$curl = curl_init('http://www.php.net');

curl_setopt($curl, CURLOPT_HEADERFUNCTION, 'dbg_curl_data');
curl_setopt($curl, CURLOPT_WRITEFUNCTION, 'dbg_curl_data');
curl_setopt($curl, CURLINFO_HEADER_OUT, true);

curl_exec($curl);

echo '<fieldset><legend>request headers</legend>
  <pre>', htmlspecialchars(curl_getinfo($curl, CURLINFO_HEADER_OUT)), '</pre>
</fieldset>';

echo '<fieldset><legend>response</legend>
  <pre>', htmlspecialchars(dbg_curl_data(null)), '</pre>
</fieldset>';

function dbg_curl_data($curl, $data=null) {
  static $buffer = '';

  if ( is_null($curl) ) {
    $r = $buffer;
    $buffer = '';
    return $r;
  }
  else {
    $buffer .= $data;
    return strlen($data);
  }
}