PHP 5.3 is not maintained anymore since 2014. We still have backports from RHEL, but supporting this version of PHP slows us down in our developments.
Therefore PHP 5.3 will not be supported anymore after Tuleap 9.15 (ETA January 3rd 2018).
We plan to switch to PHP 7, but the gap is too much important. PHP 5.6 is a good necessary step (with an eol from PHP project at the end of 2018).