Check out example codes for "how to ckeck that email is present in databse in mysqli". It will help you in understanding the concepts better.

Code Example 1

$select = mysqli_query($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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