A variable is $ plus a name starting with a letter or underscore, and its name is case-sensitive ($price is not $Price). Assignment creates the variable and copies the value; & makes a reference, a second name for the same value (Functions and Closures).
<?php
$price = 10;
$copy = $price; // assignment copies the value
$alias = &$price; // a reference: a second name for the same value
$price = 12;
$field = 'title';
$$field = 'PHP Basics'; // a variable variable: this creates $title
echo "$price $copy $alias | $title | ", ${'ti' . 'tle'}, "\n";
define('TAX_RATE', 0.08); // a function call: runs when execution reaches it
define('TAX_RATE', 0.1); // a constant cannot change
echo $discount; // an undefined variable warns and reads as null12 10 12 | PHP Basics | PHP Basics PHP Warning: Constant TAX_RATE already defined, this will be an error in PHP 9 in /tmp/ch04-02/variables.php on line 10 PHP Warning: Undefined variable $discount in /tmp/ch04-02/variables.php on line 11
A variable variable takes its name from another variable's value; an array such as $data['title'] does the same job visibly. "${name}" inside a string is deprecated since 8.2 in favor of "{$name}". A constant has no $, never changes and is visible everywhere. const CURRENCY = 'USD'; is compiled, so it may appear only at top level or in a class; define() runs at run time and can sit inside an if. Since PHP 8.0 an undefined constant throws Error instead of being read as a string, and case-insensitive constants are gone.