Help me understanding this source code of a Phaser
_zm1 is the internal state variable of the allpass. The allpass is an IIR filter and hence it's current output is dependent on past inputs and outputs. This is the way to keep track of this. 

In this particular case it's implemented using a set of coupled difference equation in so-called "Transposed Form II" filter topology. So $zm_1[n] = y[n]*a_1+x[n]*b_1 $ . For a first order allpass we have $b_1 = 1$