PHP scripts must be located in htdocs

The file extension is .php

Code is protected by <? ... ?> 

A field entitled "something" produces a variable 
"$something" that contains the value brought in by 
the field (this is the CGI convention, e.g., in Perl
with CGI.pm we would have said that a field entitled
"something" can have its value accessed through the
following sequence: 

  use CGI; 

  $q = new CGI; 

  $something = $q->param('something'); 

In PHP we get this without any effort.)

So we start from this combination of files (in htdocs/0218): 

   UW PICO(tm) 4.10                     File: form.html                                   

<form action="one.php">
  Name: <input type="text" name="who"> <br>
  Age: <input type="text" name="age"> <p>
  Press <input type="submit" value="Proceed"> to move on.
</form>


   UW PICO(tm) 4.10                      File: one.php                                    

Well <? echo $who ?> you will be <? echo $age + 1 ?> years old next year.

And we can combine the two:

   UW PICO(tm) 4.10                      File: one.php                                    

<? if ($who) { ?>

Well <? echo $who ?> you will be <? echo $age + 1 ?> years old next year. <p>

<? } else { ?>

Please use the form below to enter and submit your data. <p>

<? } ?>

<form action="one.php">
  Name: <input type="text" name="who"> <br>
  Age: <input type="text" name="age"> <p>
  Press <input type="submit" value="Proceed"> to move on.
</form>

And we have two examples: 

1. Simple counter with two buttons state client side hidden fields. 

   UW PICO(tm) 4.10                      File: two.php                                    

<?
  // retrieve state gives us $balance, $times for free
  if ($times) {
    if ($direction == "up") {
      $balance += 1;
    } else {
      $balance -= 1;
    }
    $times += 1; 
  } else { // initialize state
    $times = 1;
    $balance = 0;
  }
 ?>
<h1> <? echo $balance ?> </h1>
You have pushed the buttons in this form <? echo $times ?> times thus far. <p>
<form>
  <input type="hidden" name="balance" value="<? echo $balance ?>">
  <input type="hidden" name="times" value="<? echo $times ?>">
  <input type="submit" name="direction" value="down">
  <input type="submit" name="direction" value="up">  
</form>

2. Same program with server-side state: 

<? session_start();
  
  // retrieve state gives us $balance, $times for free
  if ($times) {
    if ($direction == "up") {
      $balance += 1;
    } else {
      $balance -= 1;
    }
    $times += 1;
  } else { // initialize state
    session_register("times");
    session_register("balance"); 
    $times = 1;
    $balance = 0;
  }
 ?>
<h1> <? echo $balance ?> </h1>
You have pushed the buttons in this form <? echo $times ?> times thus far. <p>
<form>
  <input type="submit" name="direction" value="down">
  <input type="submit" name="direction" value="up">  
</form>

The session is in:

-bash-3.2$ cd apache/
-bash-3.2$ cd phpsessions/
-bash-3.2$ ls
-bash-3.2$ ls -l
total 8
-rw------- 1 dgerman faculty 22 Feb 18 12:28 sess_04afcc6d592c96e51c378cf785876648
-bash-3.2$ cat sess_04afcc6d592c96e51c378cf785876648 
times|i:3;balance|i:2;-bash-3.2$ cat sess_04afcc6d592c96e51c378cf785876648 
times|i:7;balance|i:-2;-bash-3.2$