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Functions  ›  Lesson

Constants

Constants 11 minA constant is a named value that is set once with const or define, has no dollar sign, cannot be changed afterwards, and can be read everywhere including inside functions.

Constants

Pageturn charges 20% VAT on every sale, and the shop's name appears on every page. If those values live in ordinary variables, one careless $vat = 0; deep in a template can change the price of everything. Values that must never change belong in constants.

The rule

A constant is set once, has no $, and is visible everywhere.

Two ways to create one:

<?php
const VAT_RATE = 0.20;              // at the top level of a file (or in a class)
define("SHOP_NAME", "Pageturn");    // a function call; works anywhere

function withVat(float $price): float {
    return $price * (1 + VAT_RATE);  // no global needed
}

echo SHOP_NAME;           // Pageturn
echo withVat(10);         // 12

The key properties:

  • No dollar sign. You write VAT_RATE, not $VAT_RATE. By convention names are UPPER_SNAKE_CASE.
  • Set once. A second define("SHOP_NAME", ...) does not change the value; PHP issues a warning ("already defined") and keeps the original. Assigning with = is not possible at all.
  • Global visibility. Unlike variables, constants can be read inside any function without passing them in. That is safe precisely because nobody can change them.
  • Case-sensitive. VAT_RATE and vat_rate are different names.

const or define? const is checked when the file is compiled, so it must sit at the top level (or inside a class), never inside an if or a function. define() runs like any other call, so it can be used conditionally, for example after reading a configuration file. Prefer const for fixed values.

PHP also ships built-in constants you will use often: PHP_EOL (the line ending), PHP_INT_MAX (the largest integer), PHP_VERSION. And there are "magic" constants whose value depends on where they appear, such as __LINE__ (the current line number) and __DIR__ (the folder of the current file).

The shortcut

For every value, ask "should anything ever be allowed to change this while the script runs?" If not, make it a constant.

Sorting Pageturn's valuesStep 1 of 4

  1. VAT rate

    Fixed by law for the whole request. Constant: const VAT_RATE = 0.20;

Drills

Answer fast — trust the rule.

quick drill

How do you read a constant named VAT_RATE?

quick drill

What happens when a script calls define("SHOP_NAME", "Other") after SHOP_NAME is already defined?

quick drill

Which way of creating a constant can appear inside an if block?

quick drill

Which value should be a constant rather than a variable?

Remember

  • A constant is set once, has no $, and is visible everywhere.
  • Shortcut: "should anything change this while the script runs?" No → constant.
  • Trap: const cannot go inside an if or a function; use define() there.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainVariable or constant?

Pageturn stores the VAT rate in $vat and uses global $vat; inside several functions. Explain why a constant is a better fit, how you would declare it, and why it can be used inside functions without passing it in.