Someone

  $query = 'SELECT * FROM user where username=\''.mysql_real_escape_string($username).'\' or email=\''.mysql_real_escape_string($email).'\'';
  $result = mysql_query($query) or die(mysql_error()) ;
  if(mysql_num_rows($result))
    {    
     //result already exists
     $available = 1;
     echo "exists";
    }
    else {
      //user is new to system
      $available = 0;
      echo "new to system";
    }