Thank you for your bug report.

Please try setting phar.readonly to false. Does this solve the problem?

I tried setting phar.readonly to false, I no longer get the exception but there is still no phar created on the specified directory.

Could you please verify that writing to /tmp works at all (for example file_put_contents() in the same script) and also say if it's 32bit or 64bit

1)file_put_contents (works!)
$strFile = '/tmp/myFile.txt';
$strData = 'hello world';

$res = file_put_contents($strFile, $strData);

if($res):
    print 'it works';
else:
    print 'it failed';
endif;

2)uname -a
Linux pegasus 2.6.27.29-0.1-pae #1 SMP 2009-08-15 17:53:59 +0200 i686 i686 i386 GNU/Linux

Could you please to run your example script again, with error reporting enabled and check whether it works? If it does not, please provide an strace, like this:

strace php -f foo.php 2> strace.txt

Please provide an URL to the strace here (you can use a pastebin). Thank you.

It works, the phar file gets created. Thanks a lot. I am trying to go through the changes to see what caused it to work.....

Setting to "Bogus" because it seemed to work after all.

After one year of reporting this bug, I got the same problem. I followed 
all comments but for me, the phar is not there!!

    $phar = new Phar('./test.phar', 0, 'test.phar');
    $phar->startBuffering();
    $phar['test.txt'] = 'phar is here';
    $phar->setStub($phar->createDefaultStub('index-cli.php', 'index.php'));