Source Code Fundamentals: Variables

A variable is a named area of data storage that has a type and a value. Distinct variables may have the same name provided they are in different scopes. A constant is a variable that, once initialized, its value cannot be changed. Based on the context in which it is declared, a variable has a scope.

The following kinds of variable may exist in a script:

Local Variables

Except for function parameters, a local variable is never defined explicitly; instead, it is created when it is first assigned a value. A local variable can be assigned to as a parameter in the parameter list of a function definition or inside any compound statement. It has function scope.

Consider the following example:

function do_it(bool $p1): void {  // assigned the value true when called
  $count = 10;
  if ($p1) {
    $message = "Can't open master file.";

Here, the parameter $p1 (which is a local variable) takes on the value true when do_it is called. The local variables $count and $message take on the type of the respective value being assigned to them.

Consider the following example:

<?hh // strict

namespace Hack\UserDocumentation\Fundamentals\Variables\Examples\LocalVar;

function f(): void {
  $lv = 1;
  echo "\$lv = $lv\n";

function main(): void {
  for ($i = 1; $i <= 3; ++$i)
$lv = 1
$lv = 1
$lv = 1

Unlike the equivalent function static version, this function f outputs "$lv = 1" each time.

Array Elements

An array is created via a vec-literal, a dict-literal, a set-literal, using array, or the array-creation operator. At the same time, one or more elements may be created for that array. New elements are inserted into an existing array via the simple-assignment operator in conjunction with the subscript [] operator.

The scope of an array element is the same as the scope of that array's name.

$colors1 = vec["green", "yellow"];   // create a vec of two elements
$colors1[] = "blue";                 // add element 2 with value "blue"
$colors2 = dict[];                   // create an empty dict
$colors2[4] = "black";               // create element 4 with value "black"
$colors3 = array();                  // create empty array
$colors3 = ["red", "white", "blue"]; // create array<string> with 3 elements
$colors3[] = "green";                // insert a new element 3

Instance Properties

These are described in the class instance properties section. They have class scope.

Static Properties

These are described in the class static properties section. They have class scope.

Class and Interface Constants

These are described in the class constants section. They have class or interface scope.