Wireless systems have become an increasingly pivotal part of our lives. Various critical applications and use cases such as healthcare, financial transactions, e-commerce, transportation, industrial automation, etc. rely on secure and reliable communication for their proper operation. Despite their widespread adoption, conventional cryptographic security mechanisms are unable to scale with the increasingly decentralized and heterogeneous networks. Physical layer security (PLS), on the other hand, provides a promising complementary solution to ensure authenticity, confidentiality, integrity, and availability of legitimate transmissions by exploiting the dynamic characteristics of the wireless environment. Despite the plethora of literary works regarding different facets of PLS being present, a unified framework is still absent. In this paper, we provide a PLS framework that not only encompasses the existing works but also enables the development of next-generation PLS methods. In line with this, the importance of PLS for emerging technologies such as joint sensing and communication, vehicular communication, non-terrestrial networks, millimeter-wave, terahertz communication, etc. is highlighted. Furthermore, the key challenges and directions for future PLS mechanisms are identified.