What is the difference between client-side and server-side programming?

javascript php client-side server-side

PHP Snippet 1:

<script type="text/javascript">
    var foo = 'bar';
    <?php
        file_put_contents('foo.txt', ' + foo + ');
    ?>

    var baz = <?php echo 42; ?>;
    alert(baz);
</script>

PHP Snippet 2:

<script type="text/javascript">
    var foo = 'bar';

    var baz = 42;
    alert(baz);
</script>