PHP Simple HTML DOM Parser would be a great place to start and the also read up on Cronjobs

But show us what you got so far, so we can help you, we are not going to write the code for you.

Edit:

the problem is with this line:

$html = file_get_contents($url);

where is the $url defined? it looks like it is empty, you can try and replace it with the website link so it looks like this.

$html = file_get_contents("http://domain.tld/page");