PHP Constants

A constant is like a variable that cannot be changed. You set it once, and from then on it stays the same for the whole script.

Use a constant for values that should never change: a tax rate, the name of your site, a maximum number of login attempts.

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<?php
const TAX_RATE = 0.18;

$price = 1000;
$tax = $price * TAX_RATE;

echo "Tax: " . $tax;

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Tax: 180

What makes constants different

• There is no $ in front of the name.
• By habit, names are written in UPPER_CASE.
• A constant can be used anywhere: inside functions and classes, with no global keyword.
• Once set, it cannot be changed or removed.

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<?php
const SITE = "Code Corner";

function welcome(): string
{
    return "Welcome to " . SITE; // no global needed
}

echo welcome();

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Welcome to Code Corner

What happens if you try to change one

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<?php
const LIMIT = 5;
echo LIMIT;

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5

Writing LIMIT = 10; later is a parse error. PHP refuses to run the file at all. Something that should not change is protected by the language itself.

Built-in constants

PHP already comes with many constants.

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<?php
echo PHP_INT_MAX . PHP_EOL;
echo PHP_INT_SIZE;

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9223372036854775807
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PHP_INT_MAX is the largest whole number PHP can store on a 64-bit system, and PHP_INT_SIZE is the number of bytes an integer uses. PHP_EOL is the line ending of your operating system, so it is the safe way to end a line in command-line scripts.

The next two lessons show the two ways to create a constant: const and define().