PL/php is a procedural language add-on for PostgreSQL.

It allows you to create PHP functions that are stored inside a PostgreSQL
database server.  They can then be used as stored procedures.  They have
many of the functionalities of a standard PostgreSQL function, and you
can use them to write triggers, set-returning functions, and plain functions
that would just be too complex or cumbersome to write in PL/pgSQL.