Shortcode is running in page editor

Question

I am trying to make a plugin that allows users to add a shortcode to their website. The problem that I am facing is that once the shortcode is placed in a page the shortcode is being run on both the page editor and on the front end, where I want it to only run on the front end.

I followed this guide on how to make a shortcode. I searched around a bit on the web and saw where someone suggested adding the function ob_start(); but it did not seem to help. I have looked at other guides and no one seems to cover this issue. What am I doing wrong?

Code:

add_shortcode('sss-redeem', 'Redeem');

function Redeem(){
    ob_start();
    $IsAdmin = is_super_admin(get_current_user_id());       
    ?>
    <form>
    <center>
    <?php
    if($IsAdmin){
        ?>
        <input type="checkbox" name="overwrite" value="true">Overwrite?<br>
        <?php
    }
    ?>
    <strong><h2>Order ID</h2></strong> <input type="text" maxlength="4" name="oid"/><br>
    <input type="text" name="pgs" value="a5b6n66f34ka1n1d" style="display: none;" />
    <button type="submit">Submit</button>
    </form>
    </center>   
    <?php
}

NOTE: I am new to developing WordPress plugins. In fact, this plugin here is my first attempt at making a plugin.

Answer 1

Thanks to @mmm I figured it out. What I am doing now is adding the form in between php tags.

EXAMPLE of what I was doing

?>
    <input type="checkbox" name="overwrite" value="true">Overwrite?<br>        
<?php

What I need to be doing is adding the content that I want to add to the page into a string that is then returned when the function is completed. So what I would want my code to look like is

EXAMPLE of what I should have been doing

<?php
$out = '<input type="checkbox" name="overwrite" value="true">Overwrite?<br>';
return $out;
?>

Answer 2

you should return value not echoing it ,you can use something like this :

add_shortcode('sss-redeem', 'Redeem');

function Redeem(){
    $IsAdmin = is_super_admin(get_current_user_id());       
    $RedeemValue.='<form>';
    $RedeemValue.='<center>';
    if($IsAdmin){
        $RedeemValue.='<input type="checkbox" name="overwrite" value="true">Overwrite?<br>'
    };
    $RedeemValue.='<strong><h2>Order ID</h2></strong> <input type="text" maxlength="4" name="oid"/><br>';
    $RedeemValue.='<input type="text" name="pgs" value="a5b6n66f34ka1n1d" style="display: none;" />';
    $RedeemValue.='<button type="submit">Submit</button>';
    $RedeemValue.='</form>';
    $RedeemValue.='</center>';
    return $RedeemValue;
}