Can't use PHP inside script tag

Question

I'm using JavaScript to alert out an error

If a user's input wrong and I put the code below:

<?php $_POST["usernamereg"] ?> = usernamereg;

the script won't alert anymore. If I remove it, then it can alert again.

<script type="text/javascript">

var usernamereg;
var passwordreg;
var password2reg;
const regisForm =document.getElementById("reg-form");
function getValue(){
 usernamereg = document.getElementById("usernamereg").value;
passwordreg = document.getElementById("passreg").value;
password2reg = document.getElementById("pass2reg").value;
}
regisForm.addEventListener("submit", (e) => { 
e.preventDefault();
getValue();


    if (usernamereg.length > 30 || usernamereg.length < 5){
        alert("Your username must be longer than 5 and less than 30"); 
        <?php $err = 1;?>;
    }else { 

        <?php $_POST["usernamereg"] ?> = usernamereg;
        <?php $err = 0;?>
    }

    if (passwordreg.length > 30 || passwordreg.length < 8){
        alert("Your password must be longer than 8 and less than 30"); 

        <?php $err = 1;?>
    } else {

          <?php $_POST["passreg"] ?> = passwordreg;

        <?php $err = 0;?>
    }
    if (password2reg!=passwordreg){
        alert("Your re-type password doesnt match");
        <?php $err = 1;?>
    }else {
          <?php $_POST["pass2reg"] ?> = password2reg;

        <?php $err = 0;?>
    }

});
</script>  

Answer 1

You need to echo the value or you are trying to set nothing to the value of password2reg .

<?php echo $_POST["passreg"] ?> = passwordreg;

Note that this is likely not a great pattern as it is very error prone.

Answer 2

The code which you have written

<?php $_POST["usernamereg"] ?> = usernamereg;

you are trying to assign a javascript variable to PHP post variable.

This is not correct.

It would be better you could post your html form code because you don't need to set the $_POST["usernamereg"] because its already set when you submit the form.

If your username and password are correct then you just need to submit the form.

Hope this helps.