/usr/lib/php5/sessionclean is in php5-common 5.5.9+dfsg-1ubuntu4.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #!/bin/sh
# first find all used files and touch them (hope it's not massive amount of files)
[ -x /usr/bin/lsof ] && /usr/bin/lsof -w -l +d "${1}" | awk -- '{ if (NR > 1) { print $9; } }' | xargs -i touch -c {}
# find all files older then maxlifetime
find "${1}" -depth -mindepth 1 -maxdepth 1 -ignore_readdir_race -type f -cmin "+${2}" -delete
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