HELP ME! HTML form validate URL exists on submit!

I have been looking everywhere and I can't sort this!

I have an existing html form asking for a pre-existing subdomain.
I need the subdomain input field to be checked on submission and halted if it does NOT exist!

eg:

User Inputs Subdomain in field of form
Code validates that hxxp://www.domain.com/Subdomain exisits
If it exists the form is sent
If it doesn't the user is asked to enter a valid subdomain!

Please help anyone?
What PHP or javascript should I include to check the validity of a URL on submission of this form like the email validation does!

 

OK, your example given is a subdomain but a sub folder.

Is the domain your domain or a remote domain?
By it "existing" do you mean that it has a webpage there?

 

I am trying to confirm if subfolder exists at http://www.xboxgamertag.com/search/{gamertag}
Because if it does then the gamertag is valid. if the code returns an error then I can assume that the text inputed to the form field is NOT a valid gamertag.

I only want to submit the form if the gamertag entered is valid.

I can also do this by checking if a file exists at
http://avatar.xboxlive.com/avatar/{gamertag}/avatar-body.png

If it does then again the gamertag is valid. if the code returns an error in that the file is not reachable then I can assume that the text inputed to the form field is NOT a valid gamertag.

How can I validate the inputted text in the form field to one of these items??
I can then either pass the form through or tell the user to enter a valid gamertag

Hope you get me!

 

You will need to do it server side rather than client side.

Would be a simple case of a short script to send a request for the headers at the URL and check for a 200 response coming back. I don't do PHP but the following seems to be the basis for it:
http://php.net/manual/en/function.get-headers.php

 

I have found a way of this to work...

<?php
function remoteFileExists($url) {
$curl = curl_init($url);

//don't fetch the actual page, you only want to check the connection is ok
curl_setopt($curl, CURLOPT_NOBODY, true);

//do request
$result = curl_exec($curl);

$ret = false;

//if request did not fail
if ($result !== false) {
//if request was ok, check response code
$statusCode = curl_getinfo($curl, CURLINFO_HTTP_CODE);

if ($statusCode == 200) {
$ret = true;
}
}

curl_close($curl);

return $ret;
}
$exists = remoteFileExists('http://www.xboxgamertag.com/search/gamertag');

if ($exists) {
echo 'Gamertag VALID Please complete form';
} else {
echo 'Gamertag INVALID Please check and try again';
}

?>

All I need to do now is replace the word 'gamertag' for the string in my form which is $userlogin
I know its basic but how do I place the string in this arguement.
I have tried {$userlogin} .$user_login. 'echo $user_login' and various other things.
I have also tried replaceing the whole url 'http://www.xboxgamertag.com/search/gamertag' with a $testurl
and had a $testurl = 'http://www.xboxgamertag.com/search/gamertag' before it but the remoteFileExists arguement doesnt like it and keeps returning as valid

Any advice?

 

The lower half should be:

$link = "http://www.xboxgamertag.com/search/{$user_login}";
$exists = remoteFileExists($link);

if ($exists) {
echo "<font color='green'>Gamertag <b>VALID</b> Please complete form</font>";
} else {
echo "<font color='red'>Gamertag <b>INVALID</b> Please check and try again</font>";
}

This works great! Trial and error, hope this helps someone else!