Values and Types

Booleans and Null

Booleans represent true-or-false answers, while null marks a missing value.

null `null` means a variable has no useful value yet.

Booleans and Null

stock
booleans_null.php
Replay: real traced execution (multi-file project)
<?php
$stock = 0;
$hasStock = $stock > 0;
$label = null;

if ($hasStock) {
    $label = "ready";
} else {
    $label = "empty";
}

echo "stock=" . $stock . "\n";
echo "hasStock=" . ($hasStock ? "true" : "false") . "\n";
echo "label=" . $label . "\n";

<?php
$stock = 2;
$hasStock = $stock > 0;
$label = null;

if ($hasStock) {
    $label = "ready";
} else {
    $label = "empty";
}

echo "stock=" . $stock . "\n";
echo "hasStock=" . ($hasStock ? "true" : "false") . "\n";
echo "label=" . $label . "\n";

<?php
$stock = 5;
$hasStock = $stock > 0;
$label = null;

if ($hasStock) {
    $label = "ready";
} else {
    $label = "empty";
}

echo "stock=" . $stock . "\n";
echo "hasStock=" . ($hasStock ? "true" : "false") . "\n";
echo "label=" . $label . "\n";

  1. $stock ← 0, $hasStock ← (empty), $label ← NULL

    1<?php2$stock→ 0 = 0; //@stock=2, 53$hasStock→ (empty) = $stock0 > 0;4$label→ NULL = null;56if ($hasStock) {7    $label = "ready";8} else {9    $label→ empty = "empty";10}1112echo "stock=" . $stock0 . "\n";13echo "hasStock=" . ($hasStock(empty) ? "true" : "false") . "\n";14echo "label=" . $labelempty . "\n";
    outputstock=0
hasStock=false
label=empty

  1. $stock ← 2, $hasStock ← 1, $label ← NULL

    1<?php2$stock→ 2 = 2;3$hasStock→ 1 = $stock2 > 0;4$label→ NULL = null;

  2. $label ← ready

    6if ($hasStock1) {7    $label→ ready = "ready";8} else {

  3. echo "stock=" . $stock . " ";

    12echo "stock=" . $stock2 . "\n";13echo "hasStock=" . ($hasStock1 ? "true" : "false") . "\n";14echo "label=" . $labelready . "\n";
    outputstock=2
hasStock=true
label=ready

  1. $stock ← 5, $hasStock ← 1, $label ← NULL

    1<?php2$stock→ 5 = 5;3$hasStock→ 1 = $stock5 > 0;4$label→ NULL = null;

  2. $label ← ready

    6if ($hasStock1) {7    $label→ ready = "ready";8} else {

  3. echo "stock=" . $stock . " ";

    12echo "stock=" . $stock5 . "\n";13echo "hasStock=" . ($hasStock1 ? "true" : "false") . "\n";14echo "label=" . $labelready . "\n";
    outputstock=5
hasStock=true
label=ready

Follow the Branch

  1. $stock starts at 0.
  2. $hasStock = $stock > 0 becomes false.
  3. $label starts as null.
  4. Because $hasStock is false, the else branch sets $label to empty.
  5. The program prints stock=0, hasStock=false, and label=empty. | stock | hasStock | label | | --- | --- | --- | | 0 | false | empty | | 2 | true | ready | | 5 | true | ready |

Exercise: booleans_null.php

Reproduce label=empty, then use the pinned stock values 2 and 5 to predict hasStock and label.