Warning: session_start(): Cannot start session when headers already sent in C:Apache24htdocsecommerceadmindbconnect.php on line 2

Fatal error: Uncaught Error: Call to undefined function mysqli_connect() in C:Apache24htdocsecommerceadmindbconnect.php:8 Stack trace: #0 C:Apache24htdocsecommerceadminindex.php(11): include() #1 {main} thrown in C:Apache24htdocsecommerceadmindbconnect.php on line 8

session_start();
$servername="localhost";
$username="root";
$password="ironman@tony";
$dbname="ecommerce";
//create connection
$conn=mysqli_connect($servername,$username,$password,$dbname);
//check connection

if(!$conn){
    die("connection failed:".mysql_error());
}else{

}