The Standard Model of particle physics (SM) has been challenged in recent years by measurements from the LHCb collaboration of rare B meson decays. These processes involve a $b\to s\ell^+\ell^-$ quark level transition and are highly suppressed in the SM, happening less than once in a million decays. This suppression provides an important testing ground for the flavour sector of the SM, as these processes are sensitive to small contributions from new particles. Ratios of branching fractions containing two different lepton flavours are particularly powerful tests, being theoretically clean. Such measurements test the flavour universality of the lepton couplings. In the SM, the electroweak couplings are universal due to an accidental symmetry of the model. This property has been extensively tested, and is found so far to be consistent with SM predictions. Nonetheless, new measurements can put important constraints on extensions of the SM. This poster will present results from a recent new measurement using $B_s^0 \to\phi\mu^+\mu^-$ and $B_s^0 \to\phi e^+e^-$ decays. The poster will also present progress towards an angular analysis of the $B_s^0 \to\phi e^+e^-$ decay that, together with the measured decay rate, will provide powerful tests of the SM. The analyses are performed using proton-proton collision data collected with the LHCb experiment, which amounts to $9\,\mathrm{fb}^{-1}$ of integrated luminosity.
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