Why is my not(!) unexpected?

This question already has an answer here:

PHP parse/syntax errors; and how to solve them? 13 answers

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The lines below:

 if(strlen($username) !<13){
     if(strlen($password) !<17){

are not valid. They should use >=. eg

 if(strlen($username) >= 13){
     if(strlen($password) >= 17){

even then, I'm pretty sure you wanted to write

 if(strlen($username) <= 13){
     if(strlen($password) <= 17){

as your error messages state the rules for your username and password!

I'm not sure I've seen any C derivative language use !< and !>

ADDENDUM: You're also missing upper bound checks on email length (there is one on the wikipedia page here).

You should consider writing a validator function, decomposing those nasty nested ifs which is really difficult code to maintain.

As a starter for 10:

   if($password != $confirmpass){
        echo ("Your passwords do not match");
    return false;
   }

   // what about weird characters?
   if (empty($username) || strlen($username) < 3 || strlen($username) > 13) {
        echo "Your username must be between 3 and 13 characters.";
    return false;
   }


   // what about weird characters?
   if (empty($password) || strlen($password) < 3 || strlen($password) > 17) {
        echo "Your password must be between 3 and 17 characters.";
    return false;
   }


   // you'll want some kind of regex to validate that the email is well formed
   if (empty($email) || strlen($email) < 3 || strlen($email) > 254) {
        echo "Your email must be between 3 and 254 characters.";
    return false;
   }

   $sql ="INSERT INTO users(user,pass,email) VALUES('".$username."','".md5($password)."','".$email."')";
   mysql_query($sql);
   header('Location: Login.php');

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Your syntax is incorrect, you are trying to negate an comparison operator but this is not syntax in PHP. The only valid forms of > and < are:

>= or <=

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