convert url's to http:// ???

[newb]

how do i convert google.com or if the user inputs www.google.com to http:// or if they already have http:// it wont add the extra http:// like in vbulletin?? does anyone know how i been trying to figure it out i dont kno how to do it someone pleeze give me example pls help thanks im new at php idk where 2 start.

[trq]

[code=php:0]
if (!strstr($url,'http://')) {
  $url = 'http://'.$url;
}
[/code]

[newb]

thx. i revised it a bit so it checks if the user inputs just www.google.com or google.com

[code]
<?php
if (!strstr($url,'http://')) {
if (strstr($url,'www')) {
$url = 'http://'.$url;
} else {
    $url = 'http://www.'.$url;
  }
}
?>
[/code]