A Phar's stub is a simple PHP file. The smallest possible stub follows:
<?php __HALT_COMPILER();
A stub must contain as a minimum, the __HALT_COMPILER(); token at its conclusion. Typically, a stub will contain loader functionality like so:
<?php
Phar::mapPhar();
include 'phar://myphar.phar/index.php';
__HALT_COMPILER();
There are no restrictions on the contents of a Phar stub, except for the requirement
that it conclude with __HALT_COMPILER();. The closing PHP tag
?>
?>
In a tar or zip-based phar archive, the stub is stored in the .phar/stub.php file. The default stub for phar-based Phar archives contains approximately 7k of code to extract the contents of the phar and execute them. See Phar::createDefaultStub() for more detail.
The phar alias is stored in a tar or zip-based phar archive in the .phar/alias.txt file as plain text.