PHP 5.3 Warning: preg_grep() expects parameter 2 to be array

function _authcache_get_http_header($regex, $default = NULL) {
  // The last header is the one that counts:
  $headers = preg_grep($regex, explode("\n", drupal_set_header()));
  if (!empty($headers) && preg_match($regex, array_pop($headers), $matches)) {
    return $matches[1]; // found it
  }
  return $default; // no such luck
}

I think the line 346 should be replaced by this :
$headers = preg_grep($regex, explode("\n", drupal_get_header()));

change:
drupal_set_header() -> drupal_get_header()

My solution

authcache_1156520_2.patch work's only with pressflow installed. You should use: drupal_get_headers

The last patch fixes the errors, but it seems that the stops authcache from caching with a pressflow base.. pressflow debug spits out the following:

"Authcache prevented cachingNO_CACHE: "Page not cached."
"INFO: "Authcache.info JSON is empty!""

A bit old, but I still use pressflow and just installed Authcache.

The patches didn't work until I added '@' in front of preg_grep.

Not sure (not a php guy), but the problem went away with either the set or get header being used.

Edit: as mentioned above, if using pressflow, use the get_header:

$headers = @preg_grep($regex, explode("\n", drupal_get_header()));

Closed because Drupal 6 is no longer supported. If the issue verifiably applies to later versions, please reopen with details and update the version.