带有echo<；脚本>；不起作用

我有这个ajax代码：

<script>
    $(document).ready(function() {
        $('ul#tab li:first').html('<?php echo $channel;?>');
        $('ul#tabs li').on("click", function() {
            //        $('ul#tab li.active').html($(this).html());
            $('ul#tab li').html("");
            //$('ul#tab li.active').html($(this).html());
            var index = $("ul#tabs li").index($(this));
            $.post("../admin/ajax/ch1.php", {
                index: index
            }, function(result) {
                $('ul#tab li.active').html(result);
            });
        });
    });
</script>

并且在php中ch1.php是

<?php
$ch1tab1= file_get_contents("../channels/ch1tab1.html");
$ch1tab2= file_get_contents("../channels/ch1tab2.html");
$ch1tab3= file_get_contents("../channels/ch1tab3.html");
$ch1tab4= file_get_contents("../channels/ch1tab4.html");
$ch1tab5= file_get_contents("../channels/ch1tab5.html");
$channel = $_POST['index'];
if ($_POST['index'] == 0 ){ $channel = $ch1tab1;}
else if ($_POST['index'] == 1){ $channel = $ch1tab2;}
else if ($_POST['index'] == 2 ){ $channel= $ch1tab3;}
else if ($_POST['index'] == 3 ) {$channel = $ch1tab4;}
else if ($_POST['index'] == 4 ) {$channel = $ch1tab5;}
echo $channel; ?>

变量ch1tab4有一个代码，它不起作用，回显空白plz我该怎么办？