PHP json_encode显示null

I am running an SQL Query in PHP and putting the values into a JS variable:

<?php
$return_arr = array();
$sql="SELECT * from customer_billing group by productname ";
$rs=mysql_query($sql,$conn) or die(mysql_error());
while($result=mysql_fetch_array($rs)) {
    $return_arr[] =  $result["productname"];
}
echo json_encode($return_arr);
?>
<script type="text/javascript">
$(function() {
    var availableTags = <?php echo json_encode($return_arr); ?>
    //autocomplete
    $(".auto").autocomplete({
        source: availableTags
    });             
});
</script>

I have 3 rows with the column productname equal to:

• Integra Fibre Unlimited
• Integra Fibre Unlimited (RRP: £59.95)
• Integra Professional Web Hosting

and when i use echo json_encode($return_arr);, it displays like:

"Integra Fibre Unlimited",null,"Integra Professional Web Hosting"

it just doesn't like displaying the second one

Try:

// some code
while($result=mysql_fetch_array($rs)) {
    $return_arr[] =  utf8_encode($result["productname"]);
}
echo utf8_decode(json_encode($response));