Recent biomedical optics experiments, imaging or quantitative measurements of chemical compounds for example, are more and more sensitive to the optical characteristics of biological tissues. Artificial scattering media are used in the laboratories in order to work in reproducible, stable and well known samples. The most difficult part of the work is to obtain an adequate phase function, since the scattering and absorption coefficients can be adjusted by an appropriate concentration of the scattering particles and the addition of an absorbing dye. The most forward way to create phantoms is to use scattering spheres of equal size. If the sphere diameter which gives the desired mean cosine of the single scattering angle is not available, similarity relations may provide the necessary adjustments. However, as we show in this paper, these similarity relations may sometimes be very inaccurate and, moreover, the Mie phase function of a sphere does not match a real tissue phase function. Arridge et al, Firbank et al have suggested that a better solution would be to use a distribution of different size scattering particles in order to imitate the whole phase function, but the determination of a mixture of spheres with adequate sizes and concentrations is a difficult mathematical problem. The goal of this paper is to solve this problem. It is first shown that the extreme complexity of real biological samples can be very simply simulated by a mixture of spheres with a fractal diameter distribution. Then some simple rules, based on the knowledge of this fractal distribution, are given in order to obtain a realistic phase function with a limited number of spheres diameters.