Check if a string contains nothing but an URL in PHP

I am wondering if this is a proper way to check, if a string contains nothing but an URL:

if (stripos($string, 'http') == 0 && !preg_match('/\s/',$string)) {
  do_something();
}

stripos() checks if the string starts with "http"
preg_match() checks if the string contains spaces

If not so, I assume that the string is nothing but an URL - but is that assumption valid? Are there better ways to achieve this?

Use filter_var()

if (filter_var($string, FILTER_VALIDATE_URL)) { 
  // you're good
}

The filters can be even more refined. See the manual for more on this.

In PHP there is a better way to validate the URL:

http://www.php.net/manual/en/function.filter-var.php

if(filter_var('http://example.com', FILTER_VALIDATE_URL)) {
    echo 'this is URL';
}

To more securely validate URLs (and those 'non-ascii' ones), you can

1. Check with the filter (be sure to check the manual on which filter suits your situation)

2. Check to see if there are DNS records

   $string = idn_to_ascii($URL);
   if(filter_var($string, FILTER_VALIDATE_URL) && checkdnsrr($string, "A")){
       // you have a valid URL
   }