A non-numeric value ?

[lat9]

my experience with later versions of php7 require the casting back to a string.

function buildBranch($parent_id, $level = 0, $submenu=true, $parent_link='')
 {
         $level = (int)$level;
         $result = sprintf($this->parent_group_start_string, ($submenu==true) ? ' class="level'. (string)($level+1) . '"' : '' );
         
         if (($this->data[$parent_id])) {

You should submit a PR, then, since the code I posted is the current version on the Zen Cart v156 GitHub.