我有一个脚本
idea_slug=$_POST['idea_slug'];
$data['idea_date']=$this->db->query("SELECT * FROM td_idea WHERE idea_slug='$idea_slug'")->result_array();
$submit_date=$data['idea_date'][0]['idea_submit_date'];
$datetime1 = DateTime::createFromFormat('d/m/Y', $submit_date);
$submit_date= $datetime1->format('Y-m-d');
$submit_date = strtotime($submit_date);
$end = strtotime('+90 days', $submit_date);
现在我想将 $end 转换为 Y/m/d 格式我该怎么做?