$showdate = "$YearSelect-$MonthSelect-$DaySelect";
$sql = mysql_query("INSERT INTO Review SET Reviewer = '$reviewName',Show_Date = '$showdate',Show_Rating = '$rating',Show_Review = '$review'");
Make sure you filter out the double quotes in the variables.
When it inserts Show_Review = 'my text " here' it gives a conflict because the MySQL command uses a ". Same goes for ; and some others.
You can try this function to filter stuff:
$review = repchars($review,'review');
$reviewName = repchars($reviewName,'reviewName');
function repchars($var,$name) {
$name = mysql_escape_string(htmlspecialchars(stripslashes($var)));
return $name;
}