Today we started with this exercise: 

Write a CGI program in Python that can update a counter at 
the user's request. The counter goes up and down by one. The 
basic outline of the interface is like this: 


   +---------------------------------------+
   |   The counter is now: ..........      |
   |                                       |
   |   Press [here] to go up.              | 
   |                                       |
   |   Press [here] to go down.            |
   +---------------------------------------+


The first [here] is a link, the second is a button. 

When you're done e-mail the code to: dgerman@indiana.edu

Now let's look at the solution. Let's start with this:

#!/usr/bin/python

import cgi

print "Content-type: text/html\n\n"

(counter, message, action) = ("", "", "")

d = cgi.FieldStorage()

if d.has_key("counter"): counter = d["counter"].value
if d.has_key("message"): message = d["message"].value
if d.has_key("action"):  action  = d["action"].value

if message: 
  if action == "up": 
    counter = str(int(counter) + 1)
  else: 
    counter = str(int(counter) - 1)
  message = "Your new counter value is: " + counter 
else:
  counter = 0
  message = "Welcome, your counter is: " + str(counter)

print """
<form>
  %s <p> 

  Press <input type="submit" name="action" value="up"> to go up. <p> 

  Press <input type="submit" name="action" value="down"> to go down. 

  <input type=hidden name=counter value="%s"> 
  <input type=hidden name=message value="%s"> 
</form> 
""" % (message, counter, message)

But where's the link, and what about the labels on the buttons? 

So here's the final version: 

#!/usr/bin/python

import cgi

print "Content-type: text/html\n\n"

(counter, message, action) = ("", "", "")

d = cgi.FieldStorage()

if d.has_key("counter"): counter = d["counter"].value
if d.has_key("message"): message = d["message"].value
if d.has_key("action"):  action  = d["action"].value

if message: 
  if action == "up": 
    counter = str(int(counter) + 1)
  else: 
    counter = str(int(counter) - 1)
  message = "Your new counter value is: " + counter 
else:
  counter = 0
  message = "Welcome, your counter is: " + str(counter)

print """
<form>
  %s <p> 

  Press <a href="?action=up&counter=%s&message=%s">here</a> to go up. <p> 

  Press <input type="submit" name="action" value="here"> to go down. 

  <input type=hidden name=counter value="%s"> 
  <input type=hidden name=message value="%s"> 
</form> 
""" % (message, counter, message, counter, message)

Now let's talk about PHP. 

PHP is a different way of writing CGI scripts. 

The setup is a bit different: 

a) PHP scripts have a .php extension

b) they are located in ~/apache/htdocs

c) but they serve the same purpose and do exactly what CGI/Python (or Perl) does

Proof: 

<?
  if ($message) { 
    if ($action == "up") {  
      $counter = $counter + 1;
    } else {  
      $counter = $counter - 1;
    } 
    $message = "Your new counter value is: " . $counter;   
  } else {
    $counter = 0;
    $message = "Welcome, your counter is: " . $counter;
  }
?> 

<form>
  <?=$message?> <p> 

  Press <a href="?action=up&counter=<?=$counter?>&message=<?=$message?>">here</a> to go up. <p> 

  Press <input type="submit" name="action" value="here"> to go down. 

  <input type=hidden name=counter value="<?=$counter?>"> 
  <input type=hidden name=message value="<?=$message?>"> 
</form> 

So the approach is much simplified in PHP.