Phar requires PHP 5.2.0 or newer. Additional features require the SPL extension in order to take advantage of iteration and array access to a Phar's file contents. The phar stream does not require any additional extensions to function.
Note that a bug in the zlib.deflate stream filter fixed in PHP version 5.2.6 and newer may cause truncation of gzip and bzip2-compressed phar archives.
PHP 5.3 configured with --enable-zend-multibyte makes phar dependant on the ini option detect_unicode.