The standard opening tag for php is <?php, however there are two alternative opening tags:

<? – short for <?php
<?= – short for <?php echo

Although they may be convenient, you should never use these tags. The main reason I don’t use them is because short tags can be disabled in the php.ini, so if your application relies on them, it will fail when this feature is disabled. Recently a couple of short tags crept into (by my own mistake) one of my applications and a user reported he was getting a parse error on the last line of code in the file. After looking over the code several times I replaced the short tags with the longer version and all was well.

If you are writing portable applications I recommend that you disable short tags in your own php.ini, so in the case that you place one by mistake (as I did) you will get the error message instead of one of your users reporting it weeks later.