Codeigniter Table Library Handle Empty / Null Values

I would do something like this:

$tableData = array (
    0 => 
    array (
        'id' => 102,
        'lastname' => 'Bing',
        'title' => 'Doctor',
        'timestamp' => '2019-01-1810:17:05',
        'member_no' => null,
        'firstname' => 'Ross', //intentionally moved to show ordering
        'foobar' => 'blah' //added for example, this will be removed by array_intersect_key
    ),
);

$heading = array(
    'id' => '<strong>ID</strong>',
    'firstname' => '<strong>First Name</strong>',
    'lastname' => '<strong>Last Name</strong>',
    'title' => '<strong>Title</strong>',
    'timestamp' => '<strong>Date Submitted</strong>',
    'member_no' => '<strong>Member Number</strong>'
);

//create a default array
//this takes the keys from $heading, and makes an array with all the values as 'not available'
// ['id' => 'not available','lastname' => 'not available', ... ]
$default = array_fill_keys(array_keys($heading), 'not available');
$rows = [];

foreach($tableData as $key => $row) {
    //remove all elements with strlen of 0 (in this case 'member_no')
    $row = array_filter($row, function($item){return strlen($item);});

    //removes 'foobar' or anything that has a key not in $heading
    $row = array_intersect_key($row, $heading);

    //combine $default and $data (empty items in $row are filled in from default)
    //such as items removed by array_filter above
    //the key order will match $default, which matches $headings
    $row = array_merge($default, $row);

    $rows[] = $row;
}

foreach($heading as $key=>$value) {
    print_r(array_merge([['data'=>$value]], array_column($rows, $key)));
}

Output

Array
(
    [0] => Array
        (
            [data] => <strong>ID</strong>
        )

    [1] => 102
    //[2] => 108
    //...
)
 ....

Sandbox

I kept these separate so it would be a bit easier to read, but there is no reason you cannot do it this way.

//...
$default = array_fill_keys(array_keys($heading), 'not available');

foreach($tableData as $key => $row) $rows[] = array_merge($default, array_intersect_key(array_filter($row, function($item){return strlen($item);}), $heading));

foreach($heading as $key=>$value) print_r(array_merge([['data'=>$value]],array_column($rows, $key)));

Sandbox

I had to guess a bit on what the end result was, so I ran your original code and it gave me this:

Array
(
    [0] => Array
        (
            [data] => <strong>ID</strong>
        )

    [1] => 102
    //[2] => 108
    //...
)
....

In my code you can replace the print_r with this call $this->table->add_row([..array data..]);. Where array data is the stuff in the print_r call. You could make this a variable, but what's the point if its only used here. That eliminates a few of those loops (see below) and A few other advantages:

• key order of $headings is preserved, elements appear where they do in the $headings array, regardless of where they are in $tableData. This allows easy re-ording of the data, too, for example: you could even map this to a dynamic array, I do this in CSV files, which allows users to change the order of the headers and columns. They can even rename the headers, because the way the key => value pairing works my_key => their_key...
• Data missing from $tableData is defaulted to not available pulled in from $default, in theory you could map this manually to different things. For example: you could default timestamp to the current time by doing $default['timestamp'] = date('Y-m-d H:i:s'); right after creating it with array_fill_keys.
• Extra data in $tableData not defined in $headings is removed. Which is good for forward compatibility.
• And it's A bit easier to make sense of (once you know how it works) because there are less "transient" variables and less fiddling with the keys ect...

Basically what I am doing is giving control over to the $headings array, in your original code. You do this somewhat by looping over the keys (fields), but this complicates things later like this $rows[$key][$key2 + 1]. It leaves you open to undefined array index issues, if the data changes at a later time, such as adding a new field to the DB. The order of the output is dependent on the data in $tableData which is less intuitive (and less useful) then if it depends on $headings.

Here is an example of these issues with the original code:

//for example if your data changes to this and run your original code
 $tableData = array (
 0 => 
    array (
        'id' => 102,
        'lastname' => 'Bing',
        'title' => 'Doctor',
        'timestamp' => '2019-01-1810:17:05',
        'member_no' => null,
        'firstname' => 'Ross', //intentionally moved to show ordering
        'foo' => 'bar' //added this undefined in $headings
    ),
);

You'll get this notice and also find the last 2 elements are:

 <br />
 <b>Notice</b>:  Undefined index: foo in <b>[...][...]</b> on line <b>30</b><br />
 //... all other elements ...

 //in orignal: displayed in the order of $tableData
 //in my code: order is based on $headings, so this would be moved to the correct location
Array(
    [0] => Array (
         [data] => <strong>First Name</strong>
    )        
    [1] => Ross
)

//in orignal: $headings['foo'] is not defined so we have no label here
//in my code: this element is removed and doesn't get rendered
Array(
     [0] => Array(
           [data] => <strong></strong>
      )

     [1] => bar
)

Sandbox (Orignal Code)

While these things may never cause an issue, it highlights my point about basing the output off of $headings and not $tableData. Things tend to change, and this way if you add/remove a field from this data, you wont have to worry about it breaking this page etc...

The combination of array_fill_keys, array_intersect_key and array_merge can be used to map the headers (As I shown above) of one array to another. You can use array_combine($headings, $row) to physically swap them and you would get something like this:

[
     [
         '<strong>ID</strong>' => 102,
         '<strong>First Name</strong>' => 'Ross',
          //...
     ],
     [...]
]

It works great for CSV files (which is what I figured it out on) and anything else you need to remap the keys for.

Anyway, hope it helps you!