Is there an easy way to define a "global" if isset condition on node.tpl.php? I'm using some custom node.tpl.php files with lots of custom fields. For each field I have to write down &lt;?php if (isset($content['field_myfield'])):?&gt; SOMETHING &lt;?php endif; ?&gt;

(It gives error if that field is not set without using if isset condition.)

Is there anyway to define a 'global' if (isset...): instead of adding if condition to each field separately?
No.  Consider the case where one field is not set.  Either the global if fails and nothing is printed or you are right back to those errors.  This is one of the shortcomings of coding in the theme layer.