The common elements like header, footer and navigation bar could simply be plain html files (with .php extensions) and inserted into their respective locations in each web page using include()
regarding working out the functionality of your website, the first thing you should do is storyboard your website.
after you have done that, then 1 way of implementing it, depending on your storyboard, is
1) create an input form to create a task (createTaskForm.php)
2) createTaskForm.php sends all the form data to a php script, createTask.php, which then processes the form data to create the task in the database
3) you can apply the same concept for editing tasks by creating a editTaskForm.php which drives editTask.php
4) you could also have a form, displayTasks.php, which displays all the tasks and relevent task info in a table with a row for each task. For each task you could add a link that takes you to editTaskForm.php with that taskId as input for ediTaskForm.php and a checkbox which sends the taskId to deleteTasks.php to delete that task if it is checked when the "Delete selected tasks" button is clicked.