When using the @param or @var markup and defining a type for a variable, phpdoc gives errors in it's report when the default value for a variable is NULL and it is not designated the "resource" type.  IMHO NULL should be a valid value for any variable type.

Ex:

/**
 * This is a function
 * @param string $var1 A variable with default=NULL
 * @return string
 */
function foo($var1 = NULL) {
  return $var1;
}

The above will produce the following in the report:

[function] foo          mismatch

1st parameter type "$var1" does not match the the documented type "string", possible error consider an update to "@param resource $var1 A variable with default=NULL" or "@param resource A variable with default=NULL", the variable name is optional.

I hope this is enough information.

Adam