Description:
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The symlink() function changes its behaviour depending on the ZTS define. If ZTS is not defined, all are created relative, if ZTS is enabled, the parameters are evaluated to actual absolute directories and thus are created absolute. This is different behaviour and should be at least documented in a proper way. 

Reproduce code:
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symlink("..", "testlink");

Expected result:
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Depending on enabled or disabled ZTS the symlink will be created relative (pointing to "..") or absolute (e.g. if currently in "/tmp/test/phptest" pointing to "/tmp/test"). If the link (or the directory containing it) is being moved/renamed later, the latter can lead to a broken link while the first cannot.