David from CodeXplorer:
>> The ONLY reason to use empty() is for code readability. It is the same as an IF/ELSE check.
>> So, don't bother using EMPTY in the real world.

This is NOT true.  empty() will not generate warnings if you're testing against an undefined variable as a simple boolean check will.  On production systems, warnings are usually shut off, but they are often active on development systems.

You could test a flag with
   <?php if ($flagvar)  ... ?>
but this can generate a warning if $flagvar is not set.

Instead of 
   <?php if (isset($flagvar) && $flagvar)  ... ?>
you can simply use
   <?php if (!empty($flagvar))  ... ?>

for easy readability without warnings.