`<?php
include_once "php/config.php";
$fname = mysqli_real_escape_string($conn, $_POST['fname']);
$lname = mysqli_real_escape_strin...
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You should format your code in multiline:
```php
your code here```
Error 500 is a backend error. Check the PHP and apache error log.
check it but i dont know how to fix
You need to check the error message to discover what is bad in your code.
What is written there?