[SalesForce] Error on testclass: System.NullPointerException: Attempt to de-reference a null object

Update line 13 as below

op.AccountId= a.id;

This will solve your issue. Dont forget to accept answer if it works..Tia

Your trigger is wrong. It introduces an infinite loop, so your code will crash. Instead, use before insert/before update. Also, there's no need to copy the list. Your entire trigger can be written as:

trigger ProjectHoursTrigger on Project_Hours__c (before insert, before update) {
    UpdatePayrollPeriod.updatePayrollPeriod(Trigger.new); 
}

As for the actual error, you should be checking if the fields are null first:

public class UpdatePayrollPeriod {
    public static void updatePayrollPeriod(List<Project_Hours__c> projectHoursList) {
        for (Project_Hours__c ph : projectHoursList){
            if(ph.Year__c != null && ph.Date__c != null) {
                First_Pay_Period__c p = First_Pay_Period__c.getValues(string.valueOf(ph.Year__c));
                if(p != null && p.End_of_First_Pay_Period__c != null) {
                    ph.Payroll_Period2__c = ((p.End_of_First_Pay_Period__c.daysBetween(ph.Date__c))/14)+1;
                }
            }
        }
    }
}

For code coverage purposes, make sure your unit test is initializing the Date__c field, and the Year__c field, assuming it's not a formula.

Finally, it looks like you're using SeeAllData=false (the default test isolation mode), so you should also insert a new First_Pay_Period__c record before inserting the Project_Hours__c record.

Edit: The "p" variable in your class was not, not the field itself. This is because, as the last paragraph notes, you did not insert a First_Pay_Period__c record.

Edit 2: After a DML statement, you need to query the records back from the database to see what the final saved values are:

testHours = [SELECT PayRoll_Period2__c FROM Project_Hours__c WHERE Id = :testHours.Id];
System.assertEquals(testHours.Payroll_Period2__c, 2);