In principle <? ?> and <?php ?> are the same, except that some servers the use of <? ?> is disabled (the setting is short_open_tag in php.ini, might you be interested) whereas <?php ?> is always supported.
<?= $something ?> is the equal to <?php echo $something ?> or indeed <? echo $something ?>. This syntax (<?= ?>) could be disabled on PHP versions prior to 5.4 (together with <? ?> via short_open_tag), but since 5.4 is always available, regardless of the short_open_tag setting.