PHP not displaying errors (turned on) and code is dying at some point

Question

I'm trying to create a simple registration form with PHP, storing the user data in a database after processing the form etc. But the PHP code will not work, but not display any errors either. I have turned on error reporting in the php.ini file and also have included:

1. error_reporting(E_ALL);

2. ini_set("display_errors", 1); at the top of all used php files. I have a feeling that it has something to do with my MySQLi statements. The most likely lines for error are here:

   if($_SERVER["REQUEST_METHOD"] == "POST"){
   
       // Validate username
       if(empty(trim($_POST["username"]))){
           $username_err = "Please enter a username.";
       } else{
           require_once "db_conn.php";
           // Prepare a select statement
           $sql = "SELECT id FROM estiweb_db WHERE username = ?";
   
           if($stmt = mysqli_prepare($link, $sql)){
               // Bind variables to the prepared statement as parameters
               mysqli_stmt_bind_param($stmt, "s", $param_username);
   
               // Set parameters
               $param_username = trim($_POST["username"]);
   
               // Attempt to execute the prepared statement
               if(mysqli_stmt_execute($stmt)){
                   // store result
                   mysqli_stmt_store_result($stmt);
   
                   if(mysqli_stmt_num_rows($stmt) == 1){
                       $username_err = "This username is already taken.";
                   } else{
                       $username = trim($_POST["username"]);
                   }
               } else{
                   echo "Oops! Something went wrong. Please try again later.";
               }
           }
   
           // Close statement
           mysqli_stmt_close($stmt);
       }
   

There are a few blocks of code similar to this to validate the other fields in register.php.

I have tried already to change the way the code is constructed, different processing order. Also I have tried to change the MySQLi to MySQL (I would prefer not to but it was worth a shot).

I have copied my files into a pastebin, I know I should just use small bits but I am unsure exactly where the problem is originating from. Would appreciate anyone who looks over this and can see anything wrong.

The links are:

• register.php: https://pastebin.com/PwfgCby5
• validation.php: https://pastebin.com/sbr2DEHK
• db_conn.php: https://pastebin.com/j14d8RWw