File: date_difference_class

Class Name:dateDiff
Description:class to calculate the difference between the two dates passed and return the difference in one(seconds, minutes, hours, days) of the formats specified
Constructor: 
dateDiff($firstDate,$secondDate,$interval);
This constructor sets the values of these variables and calls the checkValid function to check the validity of the dates passed to the function.

Methods:
callDiff()
calculates the difference between the two dates and returns the difference in the specified format
Pre-requisite for using the class:
Second date should be greater than the first date
Dates passed to the function should be valid