function _make_url_clickable_cb($matches) {
	$ret = '';
	$url = $matches[2];
	if ( empty($url) )
		return $matches[0];
	// removed trailing [.,;:] from URL
	if ( in_array(substr($url, -1), array('.', ',', ';', ':')) === true ) {
		$ret = substr($url, -1);
		$url = substr($url, 0, strlen($url)-1);
	}
	return $matches[1] . "<a href=\"$url\" rel=\"nofollow\">$url</a>" . $ret;
}
function _make_web_ftp_clickable_cb($matches) {
	$ret = '';
	$dest = $matches[2];
	$dest = 'http://' . $dest;
	if ( empty($dest) )
		return $matches[0];
	// removed trailing [,;:] from URL
	if ( in_array(substr($dest, -1), array('.', ',', ';', ':')) === true ) {
		$ret = substr($dest, -1);
		$dest = substr($dest, 0, strlen($dest)-1);
	}
	return $matches[1] . "<a href=\"$dest\" rel=\"nofollow\">$dest</a>" . $ret;
}
function _make_email_clickable_cb($matches) {
	$email = $matches[2] . '@' . $matches[3];
	return $matches[1] . "<a href=\"mailto:$email\">$email</a>";
}
function make_clickable($ret) {
	$ret = ' ' . $ret;
	// in testing, using arrays here was found to be faster
	$ret = preg_replace_callback('#([\s>])([\w]+?://[\w\\x80-\\xff\#$%&~/.\-;:=,?@\[\]+]*)#is', '_make_url_clickable_cb', $ret);
	$ret = preg_replace_callback('#([\s>])((www|ftp)\.[\w\\x80-\\xff\#$%&~/.\-;:=,?@\[\]+]*)#is', '_make_web_ftp_clickable_cb', $ret);
	$ret = preg_replace_callback('#([\s>])([.0-9a-z_+-]+)@(([0-9a-z-]+\.)+[0-9a-z]{2,})#i', '_make_email_clickable_cb', $ret);
	// this one is not in an array because we need it to run last, for cleanup of accidental links within links
	$ret = preg_replace("#(<a( [^>]+?>|>))<a [^>]+?>([^>]+?)</a></a>#i", "$1$3</a>", $ret);
	$ret = trim($ret);
	return $ret;
}

http://zenverse.net/php-function-to-auto-convert-url-into-hyperlink/

In wordpress, if you want to auto convert all URLs in your string into clickable hyperlinks, you can actually do it using the built-in function make_clickable(). 

If you need to do that outside of wordpress, you can use the same function.
Example:
$string = 'I have some texts here and also links such as http://www.youtube.com , www.haha.com and [email protected]. They are ready to be replaced.';
 
echo make_clickable($string);

Function to Auto Convert URL into Clickable Hyperlink (Anchor  Tag)

url, mail, link

PHP