I would differ from Raghunath because nowhere in the EPF & MP Act it is said that PF should be deducted and contributed on the total of Basic and DA, if the same exceeds Rs 15000. But the employer and or employee can very well restrict his or their contribution to 12% (or such other percentage that the government shall fix) of Rs 15000. As such, if the sum of basic and DA exceeds Rs 15000 or is Rs 17500, there is no obligation either to the employee or to the employer to contribute PF at the rate of 12% of Rs 17500 but either or both can do it on 12% of Rs 15000.
If some of the employees want to contribute on Rs 17500 or any higher amount like Rs 30000 or Rs 35000 they can do so. Even if the employees are contributing on higher salaries, there is no compulsion that the employer should also contribute his share on the same salary. At the same time, if the employer's share of contribution is a part of remuneration like that in the case of CTC wherein whatever cost that the employer incurs on an employee will be put as part of remuneration payable, then the employer can also contribute on such higher salaries. (had it been done earlier, the employee would have got the benefit of higher pension)
Simply, an establishment can have different rates of PF not lower than 12%, different PF qualifying salaries, say Rs 15000, or higher than Rs 15000, for employees and the employer. That is, there can be employees contributing on Rs 15000 and employees contributing on actual salary higher than Rs 15000. There can also be employees in respect of whom employer is contributing on a salary of Rs 15000 and employees in respect of whom the employer is contributing on actual salaries which are higher than Rs 15000.
However, it is advisable to avoid the employer contribution on different PF qualifying salaries because it will lead to disputes by those employees in respect of whom employer had restricted his contribution to 12% of Rs 15000. In fact contribution on higher salary would be as part of an understanding between the employees and the employer and the same is reflected in their CTC and the take home salary, the employees who get lower amounts as employer contribution would raise it as a dispute in future. Therefore, it is good if you follow the same practice for all the employees, either to pay it on Rs 15000 or on actual salaries. Employees can chose either 15000 or actual salary because it gives them a tax benefit by contributing to PF.