So in physics, the notation $\int \dd{x} f(x) \triangleq \int f(x) \dd{x} $, i.e. the differential is allowed to precede the rest of the integrand. This is because you can end up with some absolutely disgusting integrand which is a complicated function of several parameters in addition to the variable of integration. The idea is, when reading from left to right, to establish which variable is the integration variable as soon as possible.