if(!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)) {
exit('Invalid email address'); // Use your own error handling ;)
}
$select = mysqli_query($connectionID, "SELECT `email` FROM `game` WHERE `email` = '".$_POST['email']."'") or exit(mysqli_error($connectionID));
if(mysqli_num_rows($select)) {
exit('This email is already being used');
}
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