AFAIK PHP runs safely in multi-threaded servers. What you can't expect is that PHP handles abnormal situations caused by flaws in the external libraries that PHP links with.
When I link with the same library in a C app, this 'abnormal situation' just results in a normal return from 'gzinflate' with an error value.
So why is it a flaw in code from zlib?
Maybe you should try this script on a multi-threaded webserver. Is this also caused by flaws in external libraries?
<?php function f() { f(); }
f(); ?>
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