Using a field with no data as a condition doesn't yield expected results

I changed it to                         
$entity2 = "Totals for ".UPPER($entity);
$entity does exist.  But based on the output, it seems like the (isset($date)) is not empty but for the exact same record, it does not exists for 5 other fields.
How can that be?

Your code does not show how (or if) $date is set. It starts with "if(isset($date)) ...".

I have rearranged your indentation, and inserted comments where the problem is

if (isset($date)){
  $sql3 ="SELECT entityID, display_name FROM $table_name4 WHERE ".((int)$entity)."=entityID";
  $e_result = @mysql_query($sql3)or die (mysql_error());
  $enum =mysql_numrows($e_result);
} else {
  $enum=0;
}
 
if ($enum !=0) {
  //Create the description for the entity in option block
  while ($erow =mysql_fetch_array($e_result)) { # $erow is created here
    $entity2 = $erow['display_name'];
  }
} else {
  $entity2 = "Totals for ".$erow['display_name']; # !! $erow does not exist here
}

                                              

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Here's where date and entity get set

$sql ="(SELECT  entityID,  type, catID,cat_subID,trans_date, payeeID, amount  FROM $table_name1 WHERE trans_date BETWEEN '$startDate' AND '$endDate' ) UNION (SELECT entityID, '', '', '', '','', SUM(amount) as amount FROM $table_name1 WHERE trans_date BETWEEN '$startDate' AND '$endDate' GROUP BY entityID )
ORDER BY `entityID`, amount ASC";
$t_result = @mysql_query($sql)or die (mysql_error());
 
//get the number of rows in the result set
//check the number of t_results
$num = @mysql_num_rows($t_result);
 
if ($num < 1) {
	//if there are no t_results,display message
	$display_block = "<P><em>Sorry!No transactions.</em></p>";
 
} else {
 
	//if t_results are found,loop through them
	//and make a form selection block
	while ($row =mysql_fetch_array($t_result)){
		$id = $row['transID'];
		$entity = $row['entityID'];
		$type1 = $row['type'];
		$catID = $row['catID'];
		$cat_subID = $row['cat_subID'];
		$method = $row['methodID'];
		$account = $row['accountID'];
		$payee = $row['payeeID'];
		$date = $row['trans_date'];
		$amount = $row['amount'];
		$description = $row['trans_desc'];
//reformat the date into mm-dd-yyyy
$date = preg_replace('/([0-9]{4})-([0-9]{2}-[0-9]{2})/', '$2-$1', $date);
                                              

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I'm not sure if I completely understand your question, but I will try to help. To find out where the data disappears, we need to first find out where it exists. I suggest you insert a vardump($row) to see what data you get from the database. This is to be removed later, of course. Something like this:

        while ($row =mysql_fetch_array($t_result)){
                var_dump($row);  # debug
                $id = $row['transID'];
                                              

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Let me explain.  My query lists a list of transactions grouped by entity.  The last line of each group is the total expenses.  When I create the line with the total for each group, only two fields have actual values.  That is why I am setting them to NULL.  But instead of displaying the entity, I want it to display "Totals for Entity" where Entity is the name of the entity.

For the record with the total, I set the num var to 0 and then say if it's =0 then set the output var to "".
It works for every single field except entity. Since I'm using the same code, just copied and pasted, I don't understand why isset($date) is not showing up as not set.

This is a bit confusing. You say "...why isset($date) is not showing up as not set." Do you mean isset($date) returns false? Or is $date set, but the value is blank? There is a difference, isset() checks if the variable is ever assigned to. It returns false only if "$date = ..." is not executed.

According to a reference book I'm using, isset checks to see if a variable is set, meaning it has a value other than NULL.  My statement says if (isset($date)){ do something} else { set the num of rows var to 0}

Then I say if num = 0, then fill this output var with "".  I do this check for all fields of this particular record.  It works with all fields testing on the same $date except for entity.  So if it's using the same $date for each check, how can (isset($date)) be false for all fields except entity?

I guess your problem begins with the condition if(isset($date)), see my comments

if (isset($date)) { //this condition is always true, its so obvious you have some value in $date.

}
else
{
  //code will never come here, so you cannot set no of rows var to 0
}

First correct this then check if some other issue is there.  

One more thing,

$enum is being assigned  a value from the line "$enum =mysql_numrows($e_result)"; not as you imagined from if( isset($date)) { ... } else {$enum =0;}, so your following code does not working as you are expecting.

if($enum !=0) {
....
....
} else {
   $entity2 = "Totals for ".$erow['display_name']; # !! $erow does not exist here
}