public with sharing class TaskController {

Public String PageId{get;set;}

Public Task Task{get;set;}

public TaskController(ApexPages.StandardController controller) {

    PageId=ApexPages.CurrentPage().getParameters().get('what_id');

    System.debug('-------->WhatId'+PageId);

    Task=new Task(Whatid=PageId);

}
Public PageReference Save()
{
    insert Task;

    PageReference pg=newPageReference('https://c.eu2.visual.force.com/apex/ProjectDetails?id='+PageId);

    return pg;
}