<?php
$username = $_POST['username'];
$password = $_POST['password'];

$stmt = $conn->prepare('SELECT username FROM users WHERE password = :password');
$stmt->execute(array('password' => $password));

$usernames = array();
while ($row = $stmt->fetch()) {    
    $usernames[] = $row['username'];
}

$usernames_str = implode(', ', $usernames);

echo "<script>swal(\"Şifre aynı olan kullanıcılar\", \"$usernames_str\", \"success\");</script>";
?>