not sure i totally undertand your question but consider this:
$array = array(
'key1' => 'value1',
'key2' => 'value2',
'key3' => 'value3',
'key4' => '',
'key5' => 'value5',
);
if (isset($array['key4']))
{
// evaluates TRUE
}
if ($array['key4'])
{
// evaluates FALSE because $array['key4'] is a blank string
}