PHP类不能识别这个->方法

当试图从同一类中的另一个方法中访问我在PHP类中定义的方法时，我得到以下错误:

Parse error: syntax error, unexpected '->' (T_OBJECT_OPERATOR) in /var/www/…/TOC.php on line 57

以前，我把这些函数放在任何类的外部，只在需要的地方包含它们。将它们移到一个文件中后，起初我认为我一定遇到了这个用户遇到的问题，但是有了类的实例，我可以从另一个文件调用我的呈现方法而不会出现问题。注释掉this->printTreeArray($sectionProjects);行消除了错误。这是我的类:

<?php
class TOC{
    private function printTreeArray($sectionProjects){
        echo "var TOCnodes = ['n";
        //Print each section, with another loop to print each one's problems
        $i = 0;
        foreach($sectionProjects as $sectionProject){
            if($i != 0){
                echo ",'n";
            }
            $project = $sectionProject->getProject();
            //Get due date for mouseover text
            $due = $project->getDueDate('F jS, Y h:i A');
            $q = new ProjectProblemQuery();
            $projectProblems = $q->findByRelProjectId($project->getId());
            $pId = $i + 1;
            echo "{id: $pId, pId: 0, name: '"Project $pId'", title: '"Due: $due'", isParent: true, open: true}";
            //Print this section's problem list
            $probId = 1;
            foreach($projectProblems as $projectProblem){
                echo ",'n";
                $nodeId = ($pId * 10) + $j;
                echo "{id: $nodeId, pId: $pId, name: '"$pId.$probId'", url: '"#'", target: '"_top'"}";
                $j++;
            }
            $i++;
        }
        echo"];'n";
    }
    public function render($section){
        if(! $section instanceof Section){
            echo "$section is not an instance of Section!";
        }
        else{
            echo "<ul id='"TOCtree'" class='"ztree'"></ul>'n";
            echo "<script type='"text/javascript'">'n";
            $sectionProjects = $section->getSectionProjects();
            this->printTreeArray($sectionProjects);
            echo "$(document).ready(function () {
                    $.fn.zTree.init($('"#TOCtree'"), setting, TOCnodes);
                });";
            echo "</script>";
        }
    }

}

这是我第一次问关于SO的问题——通常我都能找到我编码困境的答案，但是这个问题让我困惑。我意识到这门课上有很多不太优雅的东西，所以如果将来有什么可以避免这种错误的风格，我非常希望能学会。