Generate a unique file name with PHP? - php

Question

I am using a php script to upload images to a directory.
I am trying to figure out why the script is not giving the images a unique filename before saving them in the directory. I am only uploading one image at a time and I have included the relevant piece of php here which I thought would create unique filenames. Can anyone tell me why the images are not getting unique names?
//proportionally resize image
private function resize_it(){
if($this->file_count > 0){
if(!is_array($this->file_data['name'])){
throw new Exception('HTML file input field must be in array format!');
}
for ($x = 0; $x < $this->file_count; $x++){
if ($this->file_data['error'][$x] > 0) {
$this->upload_error_no = $this->file_data['error'][$x];
throw new Exception($this->get_upload_error());
}
if(is_uploaded_file($this->file_data['tmp_name'][$x])){
$this->curr_tmp_name = $this->file_data['tmp_name'][$x];
$this->get_image_info();
//create unique file name
if($this->random_file_name){
$this->new_file_name = uniqid().$this->get_extension();
$this->unique_rnd_name[$x] = $this->new_file_name;
}else{
$this->new_file_name = $this->file_data['name'][$x];
}
$this->curr_tmp_name = $this->file_data['tmp_name'][$x];
$this->image_res = $this->get_image_resource();
$this->save_dir = $this->destination_dir;