that looks like:
https://github.com/danmarsden/moodle-mod_attendance/issues/279
you are using a really old version of PHP - you should be planning an upgrade to PHP 7 at some point soon (you will see some nice performance improvements with this too.) - newer versions of Moodle do not support your old PHP release so you will need to do that before you upgrade moodle next anyway.
in the meantime the 3.1 branch on github has a fix for that - grab the download for the MOODLE_31_STABLE branch - direct download here:
https://github.com/danmarsden/moodle-mod_attendance/archive/MOODLE_31_STABLE.zip