php - find if an array contains an element

I have an array with just a list of ids, like so:

$my_array = array(
 12, 17, 99, 23
);

Now I know I could probably do something like:

function in_array($haystack = array(), $needle = NULL)
{
 foreach($haystack as $id)
 {
  if ($id == $needle)
  {return TRUE;}
  else
  {return FALSE;}
 }
}

but it seems like there's probably already a function built. What could I use?