This document discusses writing PHP extensions in Go using cgo. It explains how to build a Go package as a shared library, export functions so they can be called from C, and register the extension in PHP. Some constraints of calling C from Go like lack of function-like macros and union member access are also noted. The document provides code examples and links to a sample GitHub project for building a PHP extension in Go. It raises that segmentation faults may occasionally occur during PHP shutdown, which could indicate an issue in the Go runtime.