How to add http:// if it doesn't exist in the URL

A modified version of @nickf code:

function addhttp($url) {
    if (!preg_match("~^(?:f|ht)tps?://~i", $url)) {
        $url = "http://" . $url;
    }
    return $url;
}

Recognizes ftp://, ftps://, http:// and https:// in a case insensitive way.

At the time of writing, none of the answers used a built-in function for this:

function addScheme($url, $scheme = 'http://')
{
  return parse_url($url, PHP_URL_SCHEME) === null ?
    $scheme . $url : $url;
}

echo addScheme('google.com'); // "http://google.com"
echo addScheme('https://google.com'); // "https://google.com"

See also: parse_url()

Simply check if there is a protocol (delineated by "://") and add "http://" if there isn't.

if (false === strpos($url, '://')) {
    $url = 'http://' . $url;
}

Note: This may be a simple and straightforward solution, but Jack's answer using parse_url is almost as simple and much more robust. You should probably use that one.