A custom method Binary Tree Class

the problem is I am stuck with displaying the data in a table. In the file attached you can see what I am trying to achieve... I would like to display the binary tree in a table. After lot of pain I managed to figure out how to deal with colspans.. but I don't know how to populate it correctly...The problem is with empty peers... I would like to have a node's method: $tree->nodes[2]->to_table_array(4); Where the parameter is $depth -> how deep it should go So I should get something like: $arr = array( ---[0]=>array(1), ---[1]=>array(2,3), ---[2]=>array(4,null, null, 5), ---[3]=>array(6, null, null, null, null, null, 7, 8) ); The key of the array is the table row or depth of the tree... each then, has an array of ids or null if there is no child there I can set a desired depth (up to 10) and the array must populate for each "layer"/"row" even if it has only nulls! I will nee also test case (with a table like in the attached file) and instead of icons there should be node ids or   for empty cell