file I/O: similar to C for file I/O and similar to Perl for directory
	operations (many of these commands will not work on our
	servers because they are too dangerous, so do not be surprised
	if you try to use one of them and it fails. They will work if
	you run the file from the command line, but not from within the
	browser.).
    a fairly complete list of file commands can be found
	at http://www.w3schools.com/PHP/php_ref_filesystem.asp.
    $file_handle = fopen(filename, "mode"): opens a file and returns either
	a file handle on success or false on failure.
	"r": read
	"w": write, overwrite existing file
	"a": append
	"x": write, signal error if file already exists
	--The return result of fopen shows the advantage of dynamic typing.
	  If you query the type of $file_handle using the gettype function,
	  you will find it is a resource if fopen succeeds
	  and a boolean (with the value false) if the fopen fails
    fclose($file_handle): closes a file
    reading input
        string fgets($file_handle): returns the next line of input with the
	    newline character attached or the boolean false if the read
	    fails. 
	    a) use the trim() function to strip the newline character, as 
		well as leading and trailing whitespace.
            b) fgets(STDIN): reads from stdin
	    c) there is no gets function: PHP was originally intended for
		server-side scripting from a browser, not a command line. Hence
		it was not set up to easily access a keyboard (i.e., STDIN).
 	fscanf($file_handle, format_string, variables) 
	    a) unlike C, fscanf reads an entire line of input and 
		discards the unused part
	    b) if there are too few fields provided, then the extra variables
		retain their old values
	    c) fscanf(STDIN, ...): reads from stdin. Like gets, there is no
		scanf in php
    writing output
	fprintf($filehandle, format_string, variables): writes a formatted
		string to a file
	fputs($filehandle, $string): writes a string to a file
    bool feof($file_handle): returns true if eof, otherwise false
        --you must attempt to read a line from $file_handle before feof will
	 	return false. Hence the following idiom will attempt to
		process one too many lines. In the last iteration, $line
		will be set to false because the read has failed:

		while (!feof($file_handle)) {
			$line = fgets($file_handle);
			... process $line ...
		}

		Instead use a priming read as follows:

		$line = fgets($file_handle);
		while (!feof($file_handle)) {
		    	... process $line ...
			$line = fgets($file_handle);
		}
		
    bool rename(string $oldname, string $newname): moves a file from
	$oldname to $newname and returns true/false depending on whether
	the move succeeds
    bool unlink(string $name): deletes a file and returns true/false depending
	on whether it succeeds
    bool mkdir(string $path, string $permissions): creates a directory in
	the specified path. 
	a. returns true/false depending on whether it succeeds
        b. permissions are in UNIX form and should be in 
             octal format e.g., "0644"
    bool chdir(string $path): changes to the indicated directory, returning
	true/false depending on whether it succeeds
    getcwd(): returns current directory path 

bool move_uploaded_file ( string $filename , string $destination ) 
   --checks to ensure that filename is a valid uploaded file (i.e., uploaded
	via PHP's POST upload mechanism). This check is important to make
	sure that no spoofing can occur.
   --moves filename to destination if the file is valid
   --this command will fail on our server because of the danger of 
     uploading a trojan horse file.