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

Question

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.

The best answer for this would be something like this:

function addhttp($url, $scheme="http://" )
{
  return $url = empty(parse_url($url)['scheme']) ? $scheme . ltrim($url, '/') : $url;
}

The protocol flexible, so the same function can be used with ftp, https, etc.

Scan the string for ://. If it does not have it, prepend http:// to the string... Everything else just use the string as is.

This will work unless you have a rubbish input string.