You're missing semi-colons after your javascript lines. Also, window.location should have .href or .replace etc to redirect - See this post for more information.

echo '<script type="text/javascript">'; 
echo 'alert("review your answer");'; 
echo 'window.location.href = "index.php";';
echo '</script>';

For clarity, try leaving PHP tags for this:

?>
<script type="text/javascript">
alert("review your answer");
window.location.href = "index.php";
</script>
<?php

NOTE: semi colons on seperate lines are optional, but encouraged - however as in the comments below, PHP won't break lines in the first example here but will in the second, so semi-colons are required in the first example.