how to check if the file is already uploaded into database

Code:

if(isset($_POST["submit"]))
   {
   if($_FILES['file']['name'])
   {
       $filename = explode(".", $_FILES['file']['name']);
       $a=$filename[0];
       foreach($a as $b){
       if($b==$filename[0]){
                           echo"file already uploaded";
                           }
                         }
    }
  }

i have to upload a file and insert the data into tables.i am done with that part. now,i want to check whether the file has been already uploaded or not. if so,alert should be thrown.