PHP数组复选框问题，请在提交后选中“正确”框

I am currently created a checkbox system based on each school in an array. Everything works fine except, that after submit, only the first checkboxes are checked. Example: I have 10 checkboxes, and check box number 4,5 and 6. After submit, box 1,2 and 3 are checked. I want box 4,5 and 6 to keep their selection after submit. Hope that you can imagine by problem. I am currently created a checkbox system based