Wide-bandgap (WBG) perovskites are one of the most promising semiconducting materials for high-efficiency tandem solar cells and emerging applications such as indoor photovoltaics and building-integrated photovoltaics. However, mixed-halide WBG perovskites suffer from photo-induced phase segregation, which induces bandgap inhomogeneity, enhances non-radiative recombination, and limits device stability. Here, we present a concise and integrated understanding of phase segregation by linking fundamental mechanisms with experimental characterization. We analyze the interplay among thermodynamic instability, strain-coupled carrier-lattice interactions, and electric-field-driven ion migration, and summarize key techniques for probing phase segregation across multiple length scales. Building on this mechanistic analysis, we review state-of-the-art strategies for suppressing phase segregation, including composition design, crystallization control, interface/passivation engineering, ion-migration regulation, strain modulation, and redox-mediated self-healing approaches. Finally, we outline key challenges and future directions toward achieving intrinsically stable, high-efficiency WBG perovskite solar cells. This review provides both fundamental insights and practical guidance for overcoming one of the most critical barriers to the deployment of WBG perovskite photovoltaics.