(PHP >= 5.3.0, PECL phar >= 2.0.0)
Phar::isCompressed — Returns Phar::GZ or PHAR::BZ2 if the entire phar archive is compressed (.tar.gz/tar.bz and so on)
Note:
This method requires the php.ini setting phar.readonly to be set to 0 in order to work for Phar objects. Otherwise, a PharException will be thrown.
   Returns Phar::GZ or PHAR::BZ2 if the entire phar archive is compressed
   (.tar.gz/tar.bz and so on).  Zip-based phar archives cannot be compressed as a
   file, and so this method will always return FALSE if a zip-based phar archive is queried.
  
No parameters.
   Phar::GZ, Phar::BZ2 or FALSE
  
Example #1 A Phar::isCompressed() example
<?php
try {
    $phar1 = new Phar('myphar.zip.phar');
    var_dump($phar1->isCompressed());
    $phar2 = new Phar('myuncompressed.tar.phar');
    var_dump($phar2->isCompressed());
    $phar2->compress(Phar::GZ);
    var_dump($phar2->isCompressed() == Phar::GZ);
} catch (Exception $e) {
}
?>
The above example will output:
bool(false) bool(false) bool(true)