Generate file name with creation date

I use dompdf to create and send a PDF file to my mail and at the same time save the .txt version on the server. Saving a file works as it should, but it’s not easy for them to force it to save it with a unique name. In this case, I wanted something like date-time.txt (09/06/2012_11: 43.txt)

or even better if it can have a name from the "refnr" text box as a name.

<label for="refnr"><b>Referensnummer:</b></label>
<input type="text" name="refnr" id="refnr" class="input" />

The code is as follows:

$html = '/html.php';
$filename = $dir.'/Admin/files/"date here".txt';
$dompdf = new DOMPDF(); 
$dompdf->load_html($html); 
$dompdf->set_paper('a4', 'portrait');
$dompdf->render(); 
file_put_contents($filename, $dompdf->output()); 

I tried to play with $name='myfile_'.date('m-d-Y_hia)';, but could not do the job, it just threw an error in this line every time. So now I'm here to seek guidance from you smart people :)