Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.

The scope of a variable is the context within which it is defined. PHP has a function scope and a global scope. Any variable defined outside a function is limited to the global scope. When a file is included, the code it contains inherits the variable scope of the line on which the include occurs.

Variables Scope: Global and Function

  • Global scope: A variable declared outside of any function or class is available only in the global scope;
  • Function (local) scope: A variable declared inside a function is available only within that function;
$globalVar = "I'm global";

function testScope() {
    $localVar = "I'm local";

    echo $localVar;
    //echo $globalVar; // Warning: Undefined variable $globalVar
}

testScope(); // I'm local

echo $globalVar; // I'm global
//echo $localVar; // Warning: Undefined variable $localVar

Arrow Functions Scope

Arrow functions, automatically inherit variables from the parent scope. Unlike normal anonymous functions (function() use ($x)), arrow functions don’t need the use keyword for capturing variables.

$number = 10;

$multiply = fn($x) => $x * $number; // $number is inherited automatically

echo $multiply(5); // 50

The global Keyword

If you want to use a global variable inside a function, you need to declare it with the global keyword.

$counter = 0;

function increaseCounter() {
    global $counter;

    $counter++;
}

increaseCounter();
increaseCounter();

echo $counter; // 2

Static Variables

A static variable inside a function retains its value between multiple calls. Unlike normal local variables, it is not destroyed when the function ends.

function increment() {
    static $count = 0; // initialized only once
    $count++;

    return $count;
}

echo increment(); // 1
echo increment(); // 2
echo increment(); // 3

Variable Variables

PHP allows you to use the value of a variable as the name of another variable. This is called variable variables.

$varName = "greeting";
$greeting = "Hello, world!";

echo $$varName; // Hello, world!

Source: Orkhan Alishov's notes