We will implement a binary tree using a PHP array with a simple recursive function-based traversal. We are just rewriting the functionality using one function instead of a class. Here is the code to do this:

function treeTraverse(array &$tree, int $index = 0,
int $level = 0, &$outputStr = "") : ?bool {

    if(isset($tree[$index])) {
        $outputStr .= str_repeat("-", $level); 
        $outputStr .= $tree[$index] . "
";

        treeTraverse($tree, 2 * $index + 1, $level+1,$outputStr);      
        treeTraverse($tree, 2 * ($index + 1), $level+1,$outputStr);

    } else { 
        return false; 
    }
    return null;
} 


        $nodes = []; 
        $nodes[] = "Final"; 
        $nodes[] = "Semi Final 1"; 
        $nodes[] = "Semi Final 2"; 
        $nodes[] = "Quarter Final 1"; 
        $nodes[] = "Quarter Final 2"; 
        $nodes[] = "Quarter Final 3"; 
        $nodes[] = "Quarter Final 4"; 

        $treeStr = ""; 
        treeTraverse($nodes,0,0,$treeStr); 
        echo $treeStr; 

If we look at the preceding code, we have simply modified the traversal function and converted it to a standalone function. It is a pure function as we are not modifying the actual input here, which is the $nodes variable. We will construct a string on each level and use that for the output. We can now convert most of our class-based structures to function-based ones.