Source Code Fundamentals: Names

Names are used to label variables, constants, functions, and user-defined types, among other things. A name must begin with an upper- or lowercase letter or underscore, which can optionally be followed by those same characters or decimal digits.

Local variable names, function parameter names and property names must be preceded by $. For example:

class Data {
  const int MAX_VALUE = 100;
  private int $value = 0;
  /* ... */
}
interface ICollection { /* ... */ }
enum Position: int {
  Top = 0;
  Bottom = 1;
  Left = 2;
  Right = 3;
  Center = 4;
}
function compute(int $val): void {
  $count = $val + 1;
  /* ... */
}

Placeholder Variables

Variables beginning with $_ are referred to as placeholder variables, and should be used where an assignment is structurally required but the value is unused. This is frequently used for the list intrinsic function and the foreach statement - for example:

• list($foo, $_, $bar) = baz(); if baz() returns a 3-element tuple but the middle element is unused
• foreach ($container as $key => $_) { /* ... */ } when iterating a keyed container and the values are unneeded

The Current Instance Variable

The name $this is predefined inside any instance method or constructor when that method is called from within an object context. $this is read-only and designates the object on which the method is being called, or the object being constructed. The type of $this is this.

Reserved Names

Names beginning with two underscores (__) are reserved by the Hack language.