What makes a phar a phar and not a tar or a zip?
PHP Manual

Phar file stub

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 

?>
may be included or omitted, but there can be no more than 1 space between the ; and the close tag 
?>
 or the phar extension will be unable to process the Phar archive's manifest.

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.