server.php

<?php
    $mysqli = new mysqli("localhost", "root", "Admirih123", "mreza");
    $mail = $_GET["mail"];
    $sqlprovjera = "select * from Korisnik where E_mail='$mail'";
    if($result = $mysqli->query($sqlprovjera))
    {
        $row_cnt = $result->num_rows;
        if($row_cnt==1)
        {
            echo("Korisnik je registrovan vec");
            $result->close();
        }
        else
        {
        $sql = "INSERT INTO Korisnik VALUES('".$_GET['mail']."','".$_GET['ime']."','".$_GET['prezime']."')";
        $mysqli->query($sql);
        echo("Korisnik je uspjesno registrovan.");
        }
    }
    $mysqli->close();
?>