Valency is simply defined as the combining capacity of the elements.The valency of phosphorus is +3.
Atomic number of phosphorus is 15. It's electronic configuration is :
K L M
2 8 5
To get a stable structure phosphorus can either lose 5 electrons or gain 3 electrons. But loosing 5 electrons is a very difficult task and a lot of energy will be required to do that. Therefore it is easier for a phosphorus atom to gain 3 electrons which gives it +3 valency. Here, + sign does not signifies the gain of electrons rather than it shows the requirement of electrons which can be fulfilled by the formation of chemical bonds.