用于检查url是否有效的函数

Well.. I want to check if the a url is valid.

So that they can not just enter things like hoadw, adwij£1%, but that the inserted url have to be a valid one like domain.com.

Was thinking of a function like this:

function checkurl($url) { 
   if ($url = "VALID") {  
      return true;  
   }else{  
      return false;
   }
}

You should use filter_var():

function checkurl($url)
  //if it's valid it return's the URL else FALSE
  return filter_var($url, FILTER_VALIDATE_URL);
}

PS: you must be running PHP 5.2 or later.