To check URL validity, this has been working nicely for me:

function url_valid(&$url) {
  $file_headers = @get_headers($url);
  if ($file_headers === false) return false; // when server not found
  foreach($file_headers as $header) { // parse all headers:
    // corrects $url when 301/302 redirect(s) lead(s) to 200:
    if(preg_match("/^Location: (http.+)$/",$header,$m)) $url=$m[1]; 
    // grabs the last $header $code, in case of redirect(s):
    if(preg_match("/^HTTP.+\s(\d\d\d)\s/",$header,$m)) $code=$m[1]; 
  } // End foreach...
  if($code==200) return true; // $code 200 == all OK
  else return false; // All else has failed, so this must be a bad link
} // End function url_exists