drop_down / check if file exist in my pc

[eluser]Unknown[/eluser]
how can I check on my pc with a particular file name already defined. if the person can log into the site online else or file exists it does not enter the site, step by step please who knows, I thank. another doubt is when I create a list of the input ci form_dropdown also, as I have insert an attribute directly in the option tag, and the information shall come dynamically from database.
e.g<select>
<optio value='x' atribute = '1'>1</option>
<optio value='y' atribute = '2'>2</option>
<select>