  The path integral formulation of constrained systems leads to obtain the
equations of motion as total differential equations in many variables. If these
equations are integrable then one can constuct a valid and a canonical phase
space coordinates. The path integral is obtained as an integration over the
canonical phase space coordinates. This approach is applied to obtain the path
integral for three singular systems and it is shown that in our formulation
there is no need to distinguish between first and second-class constraints, no
need for fixing any gauge, as will as no need to enlarge the phase space.
