File not uploading to the server - php

Question

I am trying to upload a file using ExtJS and PHP.
My file is not uploading to the server.
JS:
function slidersave(){
var form = sliderform.getForm();
if (form.isValid()) {
var slider_name=Ext.getCmp('txtslidername').getValue();
var slider_image=Ext.getCmp('txtsliderimage').getValue();
var slider_link=Ext.getCmp('txtsliderurl').getValue();
form.submit({
url: 'slider/file',
waitMsg: 'Uploading your slider image...',
success: function(form, o) {
var obj = Ext.util.JSON.decode(o.response.responseText);
alert(obj);
if (obj.failed == '0' && obj.uploaded != '0') {
Ext.Msg.alert('Success', 'All files uploaded');
} else if (obj.uploaded == '0') {
Ext.Msg.alert('Success', 'Nothing Uploaded');
} else {
Ext.Msg.alert('Success',
obj.uploaded + ' files uploaded <br/>' +
obj.failed + ' files failed to upload');
}
}
});
}
}
PHP:
public function file()
{
if(isset($_FILES)){
$uploaddir = '/siteimages/slider/';
$uploadfile = $uploaddir . basename($_FILES['txtsliderimage']['name']);
if (move_uploaded_file($_FILES['txtsliderimage']['tmp_name'], $uploadfile)) {
echo '{success: true}';
} else {
echo '{success: false}';
}
}
}
I am not getting anything in the response the loader just disappears after sometime.
I don't know what's happening.