如何使用out form直接将文本框值分配给codeigniter中的会话

<input type="hidden" ID="lang" Class="form-control" value="English">
<?php  $this->session->set_userdata('user_lang', 'Arabic');?>

On dropdown change the textbox value get changed i need to assign this textbox value to session without any form in codeigniter

You can use jquery to trigger the change of value. and then run an ajax to set session data

$('#lang').change(function(){
   var val = $(this).val();
   $.ajax({
       url: "<?=site_url('your_controller/your_method')?>",
       type: "POST",
       data: {val: val},
       success: function(res){
          // your code after the ajax completed
       }
   });
});

now at controller create a new method which will set session data

functoin your_method(){
   $this->session->set_userdata('user_lang', $this->input->post('val'));
}