fopen（）没有返回资源

So I have this bit of code below to determine if the $this->stream is a resource. If it is not a resource then it should use fopen to create a resource, but it doesn't seem to work.

How do I make $this->stream a resource I thought fopen() was designed to open or create a file if it doesn't exist. Anything opened or created should be a resource correct? Why would I hit the code to throw an exception?

if (!is_resource($this->stream)) {

    $this->errorMessage = null;
    set_error_handler(array($this, 'customErrorHandler'));
    $this->stream = fopen($this->url, 'a');

    restore_error_handler();

    if (!is_resource($this->stream)) {
        $this->stream = null;
        throw new \UnexpectedValueException(sprintf('The stream or file "%s" could not be opened: '.$this->errorMessage, $this->url));
    }
}