The determination of the chemical composition of extrasolar planets has been the subject of numerous studies in the last years. While the composition of the atmosphere of planets can be observed through, e.g., the absorption of the star light, the composition of the solids remains difficult to estimate. As both planetesimals and planets are formed in the same environment, their composition is expected to be the same as their host star. Elemental ratios are important as they govern the distribution and formation of chemical species in the protoplanetary disc: the C/O ratio controls the amount of carbides and silicates that can be formed while Mg/Si gives information about the silicate mineralogy. Silicates are an important ingredient in the formation of rocky planets, as they are the most abundant component of the mantle and crust of these planets. We present a uniform study of C/O and Mg/Si element abundance ratios for 101 stars with detected planets and 430 stars without detected planets obtained using high resolution spectra from the HARPS sample. We will discuss the implications of these ratios on the nature of planets that could have been formed in those planetary systems.