Description:
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The script references in the session.save-path documentation, mod_files.sh, found at https://github.com/php/php-src/blob/master/ext/session/mod_files.sh
says #!/bin/sh at the top (and in the middle), but it uses [[ extensively which is a bashism.

If your default shell isn't bash but dash, you end up with an quite deep subdirectory (0/0/0/0/0/.....) because the recursion won't stop.

Fix is quite easy: change #!/bin/sh (and the call for sh later on) to #!/bin/bash.