Question

I want it to increment and decrement the value in $_SESSION['selection'].
Mousedown on next form results in: “The session is 1“.
Mousedown on previous form results in “The session is -1“.

Here is the code:

  <?php

    if (isset($_POST['next'])) {
      displaynext();
    }
    else if (isset($_POST['previous'])) {
      displayprevious();
    }
    else {
      session_start();
      echo "session started";
      $_SESSION['selection'] = 1;
    }

    function displaynext() {
      $_SESSION['selection'] = $_SESSION['selection'] + 1;
      echo "The session is $_SESSION[selection]";
    }

    function displayprevious() {
      if ($_SESSION['selection'] != 1) {
        $_SESSION['selection'] = $_SESSION['selection'] - 1;
      }
      echo "The session is $_SESSION[selection]";
    }

    ?>

    <form action="<?=$_SERVER['PHP_SELF'];?>" method="post">
      <input type="submit" name="previous" value="Previous">
    </form>

    <form action="<?=$_SERVER['PHP_SELF'];?>" method="post">
      <input type="submit" name="next" value="Next">
    </form>

Answer 1

Put the session_start() call on the beginning of your file; right after the <?php open tag. You also need to do that on every script that you plan to use sessions.

By the way, instead of writing:

$_SESSION['selection'] = $_SESSION['selection']-1;
$_SESSION['selection'] = $_SESSION['selection']+1;

You can use

$_SESSION['selection']--;
$_SESSION['selection']++;

Read: Increment/Decrement operators