Simply put, you shouldn't be using $_POST in the function signature(or body) - you should be using it in the place that you call your function.
function checkEmail($emailAdr)
{
$possible = "/^[a-z]$/i";
if (strlen($emailAdr) >= 8)
{
if (preg_match ($possible, $emailAdr))
{
return true;
}
else
{
return false;
}
}
else
{
return false;
}
}
Followed by the call to the function:
$emailValid = checkEmail($_POST['email']);