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If you don't have server level access on the old platform, install he Display PHP (It is old but still works on WP 4.2+). When you go to the landing page of the dashboard, it tell you your PHP version there.
IMPORTANT: CHECK FOR short_tag_open = on
Pantheon DOES NOT support this WP common setting. What this does is lets devs use <? instead of <?php. Check the config/php/php.ini and see if this is et to on. If it is you have to hunt a peck and replace all the <? with <?php, FUN!
PANTHEON PLUGINS REQUIRED
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