I am writing to a txt file to back up certain posts. Here is my simple code

$this->path = APPPATH . "post_backups/";
    $string = $this->input->post('post');
    $post_title = $this->input->post('post_title');
    $this->file = $this->path . $post_title."txt"; 
    write_file($this->file, $string);
    $this->index(); 

Every thing works fine except this

$this->file = $this->path . $post_title."txt"; 

I am trying to name my file with the title of the post IE: title.txt.

What I am getting it s this "titletxt". How should the $post_title . "txt" be written to provide txt as an extension?

Thanks