You have to define the redirect_to
function before calling it. Try this code
<?php
if($result){
$num_rows = mysqli_num_rows($result);
if($num_rows == 1){
$found_user=mysqli_fetch_array($result);
redirect_to("main.php");
}
else{
redirect_to("index.php");
}
}
function redirect_to($location){
header('Location:'.$location);
}
?>
solved php redirect_to() function undefined