在文本框中显示字母数字值（不基于数据库值）

I want to display Order number which is alphanumeric value to be displayed in a textbox.Whenever the page get reloaded,the value should be automatically incremented..eg:A0001,A0002,A0003,....like this.. Pls anyone help me..

<?php
session_start();
if(!isset($_SESSION["uid"]))
{
$_SESSION["uid"]=0;
}
$prefix='A000';
$id=$_SESSION["uid"]+1;
$_SESSION["uid"]=$id;
?>
<input type="text" value="<?php echo $prefix.$id; ?>">