Determine whether a string is trying to be a URL

is_valid_url.php

<?php
/**
 * Determine whether a string is trying to be a URL
 *
 * @see http://daringfireball.net/2010/07/improved_regex_for_matching_urls
 *
 * @param string $url
 * @return bool preg_match() returns 1 if the pattern matches given subject, 0 if it does not, or FALSE if an error occurred.
 */
function is_valid_url( $url ) {
    return preg_match( "~(?i)\b((?:https?://|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'\".,<>?«»“”‘’]))~", $url );
}