PHP and HTML Logging in

wow. there are a lot of mistakes

$pass=$_POST("pass")
$uid=$_POST("uid")
$clicked=$_POST("R")

should be

$pass=$_POST["pass"];
$uid=$_POST["uid"];
$clicked=$_POST["R"];

you forgot a lot of semicolons

form should look like

<form action="login.php" method="post" style="border-color:#0068AD" >
    <input type="password" name="pass" /><br/>
    <input type="submit" action="login.php" name="L" value="Log in"/>
</form>
<br/>

(submit must be in form)

Your forms are broken. All of your inputs need to be inside one form in order to all be submitted in the same POST:

<form action="register.php" method="post">
  <input type=text name="uid" /><br/>
  <input type="password" name="pass" /><br/>
  <input type="submit" action="register.php" name="R" value="Sign Up" />
</form>

And:

<center><cc><b>Please login to proceed:</b></cc></center>
<center>
  <form action="login.php" method="post" style="border-color:#0068AD" >
    <input type=text name="uid" /><br/>
    <input type="password" name="pass" /><br/>
    <input type="submit" action="login.php" name="L" value="Log in"/><br/>
  </form>
</center>
<center><cc><a href="register.html"> Register </a></cc></center>

(Also, it's best not to use the center tag anymore. I understand that you're just getting started, but you'll want look into using CSS for your styling sooner rather than later. It will lead to much cleaner and easier to manage HTML markup.)

This is my best guess at what you were trying to do based on the original code. I would like to see inside sqlconnect.php to see what's going on in there...

Corrections are FAR too numerous to mention, ask specific questions about anything you don't understand...

Register

HTML:

<form action="register.php" method="post">
    <!-- forms should contain ONLY block level elements as direct children! -->
    <div>
        <input type="text" name="uid" /><br/>
        <input type="password" name="pass" /><br/>
        <input type="submit" value="Sign Up" />
    </div>
</form>

PHP:

$pass = $_POST["pass"];
$uid = $_POST["uid"];

if ($uid and $pass) { 
    require "sqlconnect.php";
    connection();
    $ins = @mysql_query("INSERT INTO `Users` (`id`,`pass`) VALUES ('".mysql_real_escape_string($uid)."','".mysql_real_escape_string($pass)."')"); 
    if ($ins) {
        echo "Registered! You can log in now :)";
        header('HTTP/1.1 302 Found');
        header("Location: http://".$_SERVER['HTTP_HOST'].dirname($_SERVER['PHP_SELF'])."/index2.php");
    } else echo "Error!";
    mysql_close($connection);
}

Log In

HTML:

<form action="login.php" method="post" style="border-color:#0068AD;">
    <div>
        <center><b>Please login to proceed:</b></center><br/>
        <center><input type="text" name="uid" /><br/>
        <center><input type="password" name="pass" /></center><br/>
        <center><input type="submit" value="Log in"/></center>
    </div>
</form>
<center><a href="register.html">Register</a></center>

PHP:

require "sqlconnect.php";
connection();


$pass = $_POST["pass"];
$uid = $_POST["uid"];

if (!$passcheck = mysql_query("SELECT `pass` FROM `Users` WHERE `id`='".mysql_real_escape_string($uid)."'")) die('MySQL Error');
$passcheck = mysql_fetch_assoc($passcheck);
mysql_close($connection);

if ($passcheck['pass'] == $pass) {
  header('HTTP/1.1 302 Found');
  header("Location: http://".$_SERVER['HTTP_HOST'].dirname($_SERVER['PHP_SELF'])."/index3.html");
} else echo ("Wrong username or password");

All fields and the button should be inside the same form. Don't use a different form for each one. I'm on mobile, so I can't post code. Left as community wiki so people will add it.

At first glance your SQL INSERT statement is incorrect. It should be INSERT INTO users (id,pass) VALUES ('$id','$pass'). There are also a lot of syntax errors that you'll need to look into.

Your submit button should be inside your form, otherwise it won't know what to submit. You should also review the security of your form, when interacting with the database you should always sanitise data with mysql_real_escape_string().

So it should read:

$pass = mysql_real_escape_string($_POST["pass"]);
$uid = mysql_real_escape_string($_POST["uid"]);
$clicked = mysql_real_escape_string($_POST["L"]);

You're also missing a lot of semi-colons which should go at the end of every single line as shown above.

You should also use double equal signs (==) or triple equals sign (===) when comparing statements in an if statement, so yours should read:

if($clicked> 1 || $passcheck == true)

You should also include curly braces, so the full if should look like:

if($clicked>1||$passcheck=true) {
    header = index3.html; //This should probably be header = 'index3.html'
} elseif(clicked> 1 || passcheck == false) {
    echo ("Wrong username or password");
}

I've probably missed some, but there are lots of errors.