How is phosphoryl chloride attaching an aldehyde to benzene?
It is a Vilsmeier–Haack reaction.

You are actually neglecting the effect of DMF (Dimethylformamide).

The Mechanism:

1. Formation of Iminium Cation:

The reaction of the DMC i.e. Dimethyl formamide (which is basically an amide) with $\ce{POCl3}$ produces an iminium cation. 

2. Aromatic Electrophilic Substitution Reaction:

The formed iminium cation now act as an electrophile and does electrophilic aromatic substitution as a result of which we get an aryl ketone or an aryl aldehyde. So that extra aldehyde group which you are speaking about came from the electrophilic substitution reaction of your initial structure with iminium ion.

Here I have drawn the whole mechanism for you.