PF contribution should be 12% of the gross salary. The salary shall be restricted to Rs 15000. If you pay HRA besides salary, that shall be excluded from PF contribution. The PF contribution need not necessarily be Rs 1800 but it can be less than that when you take it at 12% of salary. But salary should mean the total remuneration that an employee gets and there should not be any compromise for that.
Many employers pay PF based on the Basic salary that they fix. They may have other allowances which are paid universally to all the employees. While deciding the PF qualifying salary you should take that 'other allowances' as well as part of salary. It can be less than Rs 15000 or more than Rs 15000. If it is more than Rs 15000, the employer CAN restrict the contribution to 12% of Rs 15000. In such case the amount will be equal to Rs 1800.