Wat Langka is one of oldest pagodas in Phnom Penh funded in 1442. It was established as a sanctuary for the Holy Writings and a meeting place for Cambodian and Sri Lankan monks. The Pagoda was named in honor of these meetings. Langka Pagoda was used as a storehouse during the Khmer Rouge era and therefore manage to avoid total destruction.
The temple still plays an important role in Cambodian Buddhism and many of the monks here are highly regarded teaches.
Wat Langkar is located on Preah Sihanouk Blvd, just southwest of the Independence Monument.

You can tell when something VIPish is happening as they'll close the roads in the area, causing complete chaos. Toward the rear of the compound, by the entrance on Pasteur Street, note the beautiful cannonball flowers growing out of the tree trunks which resemble giant Venus flytraps. 

Khmers are suspicious of these flowers and allow them to grow only at temples and the Royal Palace. It is believed a broth made from their petals is a helpful tonic for pregnant women. The monastery is said to have originally housed an order from Sri Lanka -- hence the name.