<form method="post">
    <input type="text" placeholder="username" name="username" />
    <input type="password" placeholder="New Password" name="password1" />
    <input type="password" placeholder="New Password" name="password2" />
    <input type="password" placeholder="Old Password" name="oldPassword" />
</form>

<?php

$username $_POST['username'];
$password1 $_POST['password1'];
$password2 $_POST['password2'];
$oldPassword $_POST['oldPassword'];


if($password1 == $password2){
    //check oude wachtwoord of niet :p

        $password = md5($password1);
        $servername = "localhost";
        $username = "username";
        $password = "password";
        $dbname = "myDB";

        try {
            $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
            $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $stmt = $conn->prepare('UPDATE Users SET password=? WHERE username=?');
            // sql prevent
            $stmt->bindParam(1, $password);
            $stmt->bindParam(2, $username);
            return $stmt;
        }

        catch(PDOException $e) {
            echo $sql . "<br />" . $e->getMessage();
        }

        $conn = null;
} else {
    echo "Wachtwoorden komen niet overeen";
}
?>