Hasee toh Phasee, a;sp Hansi to Phansi, is a common Hindi/ Urdu phrase which means (if the girl) laughed, then you have won her.
As such hansna means to laugh and phansna means to be trapped, so the literal meaning would be If she laughed, she is trapped.
Hasee Toh Phasee is a romcom starring Siddharth Malhotra, Parineeti Chopra and Adah Sharma.